When something appears there, it is because, if I can express myself in this way, that the lack is lacking. Now this may appear to be simply a joke, a concetti which is well placed in my style which everyone knows is Gongoric. Well, I don’t give a damn. I would just like to point out to you that many things can appear which are anomalous, this is not what makes us anxious. But if all of a sudden all norms are lacking, namely what constitutes the lack ―because the norm is correlative to the idea of lack― if all of a sudden it is not lacking ―and believe me try to apply that to a lot of things– it is at that moment that anxiety begins.

☛ The Seminar of Jacques Lacan : Book X : Anxiety : 1962-1963 by Jacques Lacan, ed. and tr. by Cormac Gallagher from unedited French typescripts, the quote reproduced above comes from the very end of the third seminar given on November 28, 1962, see p. 35 of Gallagher’s translation.

Here’s the original French version:

Quand quelque chose apparaît là, c’est donc, si je puis m’exprimer ainsi, que le manque vient à manquer.
Cela pourra vous apparaître une pointe, un concetto, bien à sa place dans mon style dont chacun sait qu’il gongorise. Eh bien, je m’en fous. Je vous ferai simplement observer qu’il peut se produire bien des choses dans le sens de l’anomalie, et que ce n’est pas ça qui nous angoisse. Mais si tout d’un coup vient à manquer toute norme, c’est-à-dire ce qui fait l’anomalie comme ce qui fait le manque, si tout d’un coup ça ne manque pas, c’est à ce moment-là que commence l’angoisse. Essayez d’appliquer ça à bien des choses. (Le séminaire, livre 10 : L’angoisse, ed. by Jacques-Alain Miller, Paris: Seuil, [1962-1963]2004, p. 53; new as well as used copies of the French edition are easily available online).

Apparently, Lacan’s series of seminars on anxiety (Book X) were published in English in 2002. It is possible to order the book online at Karnackbooks.com However, the totality of the book is also available as a PDF via Dublin Business School Esource website.
Jacques Lacan seminars are not all published yet, neither in English nor in French. The story of their publication is not simple. French readers can catch a glimpse of it by visiting Pierre Assouline’s blog La république des livres: see the entry on September 7, 2011: “Miller au Seuil : finies les lacaneries !”

• • •

Wikitionary provides answers regarding concetti and gongoric. Basically Lacan is telling his audience that he’s well aware of being (in)famously renowned for his use of puns (for more about about this reputation see here: 30th anniversary of Jacques Lacan’s death).
During another of his seminars on anxiety, Lacan said:

But the total filling of a certain void which should be preserved which has nothing to do with either the positive or negative content of the demand, this is where there arises the disturbance in which anxiety is manifested. (fifth seminar, December 12, 1962, p. 58 of Gallagher’s translation)

In the original French:

Il y a toujours un certain vide à préserver, qui n’a rien à faire avec le contenu, ni positif, ni négatif, de la demande. C’est de son comblement total que surgit la perturbation où se manifeste l’angoisse. (Paris: Seuil, [1962-1963]2004, p. 80)

I cowrote something similar a couple of years ago about the characters in Wong Kar-wai’s film 2046 (2004):

La réalité a ainsi pour condition d’effectivité un vide, qui ne peut ni ne doit être comblé, que les personnages cherchent pourtant à remplir avec obstination, sinon à recouvrir au moyen des figures et des motifs du passé — c’est, semble-t-il, le sens même de leur vie. (“Wong Kar-Wai ou l’esthétique-fiction” Cinémas : revue d’études cinématographiques / Cinémas: Journal of Film Studies, vol. 15, n° 2-3, 2005, p. 63- 86; PDF)

The void or emptiness is necessary for those characters: it’s a productive force driving their life as well as Wong Kar-wai’s story (the whole film starts and ends in and out of a hole). It’s the absence of void that is problematic: it’s a threat both the existence of the characters and storytelling in general. Recently, I came across Georg Christoph Lichtenberg’s aphorism on the importance of tubes which also seems to echo, somehow, Lacan’s thoughts on voids: see here “The most important things are done through tubes”.
Lacan also explicitly refers to his aphorism in his “Preface to the English-language edition” of his next Seminar: Book XI – The Four Fundamental Concepts of Psychoanalysis:

I have done so by virtue of having produced the only conceivable idea of the object, that of the object as cause of desire, of that which is lacking.
The lack of the lack makes the real, which emerges only there, as a cork. This cork is supported by the term of the impossible —and the little we know about the real shows its antinomy to all verisimilitude. (ed. Jacques-Alain Miller, tr. by Alan Sheridan, New York: W.W. Norton & Company, 1981, p. ix)

As it’s usually the case with aphorisms, one can be tempted to interpret Lacan’s “lack of lacking” accordingly to one’s own desires (just as I may have done myself above). For some guidance, one can read about three ways to think about this aphorism in Roberto Harari’s book Lacan’s seminar on “anxiety”: an introduction (Other Press, LLC, 2001). See specifically chapter 3: “To Lack Lack, a-Thing, Acting Out, and Passing to the Act”.