When I have fears that I may cease to be
Before my pen has glean'd my teeming brain,
Before high piled books, in charact'ry,
Hold like rich garners the full-ripen'd grain;
When I behold, upon the night's starr'd face,
This line expresses one of Keats’s biggest fears, that he would die before having the opportunity to commit all of this thoughts to paper.  Gleaning  originally meant collecting leftover crops from fields that had already been harvested. This was a laborious task in which Keats sees as analogous to writing poetry. In  ‘Sleep and Poetry’ , he says he requires ten years so “that [he] may overwhelm [himself] in poesy”. 
 Keats' writing is typically dense and concise. the phrase ‘as glean’d my teeming brain’ is a good example. Also, ‘glean’d’ introduces the  Pastoral  theme which appears in many of his poems. 
 
 Keats also fears that if he dies prematurely, his intelligence and love for writing will not be successfully developed, rendering him forgettable in the history of influential writers. Ironically, it is this fear, expressed here, that ensured his legacy.