Keats is referring to a. his completed poems.
In "When I Have Fears That I May Cease to Be", Keats talks about his fear of death--but more than that, he describes his fear of not being able to achieve everything he wants to before he's gone. He describes it in the passage:
When I have fears that I may cease to be
Before my pen has gleaned my teeming brain,
Before high-pilèd books, in charactery,
Hold like rich garners the full ripened grain;
The high-piled nooks represent his complete work, the poems he has written, while the "Hold like rich garners the full ripened grain" make reference to the full potential and recognition he would like them to reach.
