He's referring to the Epic Cycle. The idea of a cyclic series of hexameter poems on the same basic heroic theme was already current probably in the fifth century (Herotodus knows about the Cypria and the Epigonoi), but Aristotle defined it clearly in writing. In the Poetics Aristotle defined the Epic Cycle as being those epics not written by Homer, who to Aristotle was a real figure. Since then various scholars defined it slightly differently, usually including the Iliad and the Odyssey and sometimes Hesiod and others. At the very least we can say that the Epic Cycle was a collection of hexameter epics that, when strung together, constructed a coherent, continuous narrative that extended more or less from the creation of the world to the end of the Heroic Age. None of these epics survive in their entirety, and some are known only from one or two lines. Within the Epic Cycle we know of two sort of sub-cycles, the Trojan Cycle and the Theban Cycle. The Trojan Cycle included, besides the two Homeric Poems, the Cypria, the Aithiopis, the so-called "Little Iliad, the Iliou Persis ("sack of Troy"), the Nostoi, and the Telegoneia. The Theban Cycle included at least the Oidopeia, the Thebais, and the Epigonoi, which relate the story of Oedipus and the Seven. Along with these was some sort of theogony and a Titanomachy.

The authorship of these poems and their value was rather debated in antiquity. Homeric authorship was ascribed to more than just the Iliad, Odyssey, Homeric Hymns, and Βατραχομυομαχία that the Alexandrian scholars accepted, but from very early on it was disputed. Herodotus flatly rejected Homeric authorship of the Cypria, and did not think the Epigonoi was Homeric either. The poems were early, mostly if no wholly late-Archaic, but to what degree (if any) their composition was oral is very unclear--the subject matter is probably Homeric or even pre-Homeric, but the verse appears to be written, not oral. They were certainly not "genuine" Homeric works, no matter how we define "Homeric." An alternate version of the beginning of the Iliad preserved by Aristoxenus, the opening line of the Epigonoi (all that has survived of it), and possibly the last book of the Odyssey (the Alexandrians ended the Odyssey at Book 23, and the last book may more properly be the beginning of the Telegony) suggest that these poems were at least sometimes linked together in a series directly, meant to be read in sequence. By the time of the Alexandrian scholars Homeric authorship of most of the poems had been firmly rejected. Instead, a string of lesser poets were said to be the authors of the Epic Cycle: Eumelus, Arctinus, Lesches, Eugammon, etc. The quality of the accompanying works in the Epic Cycle (which were certainly composed after the Homeric Poems and some of which were quite short) was pretty unanimously recognized as being lesser than Homer. Compared to the truly enormous quantity of quotations from Homer (or local variations thereof) the poems of the Epic Cycle are hardly quoted at all, and the total lack of papyri (compared to the not inconsiderable quantity of Homer and Hesiod) containing any of the poems of the Epic Cycle suggest that by the end of the fourth century or so the poems were effectively no longer being read. Aristotle, in defining the Epic Cycle, derided their quality as inferior to Homer's. Of the Cypria and the Little Iliad he said there was no overarching theme, plot, or characters, and was totally all over the place. Other authors voiced similar complaints, and many of the poems seem to have been composed basically to fill in narrative gaps rather than actually to do anything. The New Pauly actually describes the poems of the Epic Cycle not as real epic poems but as "mere series of facts in verse (‘and then ─ and then’)," which is basically the opinion of the Alexandrian scholars, and notes that "compared with the epics of the EC, the outstanding quality of Homer becomes apparent."

While the quality of the Epic Cycle's verse and organization was probably inferior to Homer's, the poems remained a pretty good resource for other poets, playwrights, and of course mythographers of the sort that popped up in the Hellenistic Period. The tragedians borrowed their plots heavily from the Epic Cycle, although it's hard to say in any particular passage whether the poet invented something or got it from an alternate tradition in a lost poem. Likewise, later poets adapted the material of the Epic Cycle or referred to it. Thus Virgil wrote about the sack of Troy, not included in Homer, and Ovid adapted parts of the Little Iliad and Cypria.