"Them" referred to a race of unnamable beings, greater than any other in existence, including the Q Continuum. "They" were responsible for the creation of the universe, and for the creation and destruction of several previous universes they found insufficiently interesting or otherwise unworthy.
"They" created an artificial world known to the Federation as Gorsach IX, where visitors had to overcome a number of challenges in order to gain an audience with "Them". In 2380, Jean-Luc Picard (with the assistance of Q), found these beings, and was put in the position of representing the entire universe, and its worthiness to continue. Recognizing the absurdity of his situation, Picard laughed -- a reaction "They" deemed interesting enough to allow the universe to survive. (TNG novel: Q & A)