Like the other responses state, the speaker is afraid of his own impending mortality. The poem is a meditation of the speaker’s feelings about death.
In the first four lines specifically, the speaker is referring to his vocation. This is where it is tempting to conflate the poet with the speaker, because Keats uses words like “pen” and “books” that are associated with writers. However, this could be instead interpreted as an artist’s fear: that he or she may not be able to express all they need to before his or her time on earth ends. The speaker’s “teeming brain” has so many ideas and emotions that he worries it is impossible to get them all out before the end of his life.
He could also be describing the “high-piled” library full of “ripened grain” as all the art he wants to consume before his death. Besides just writing, the speaker appreciates the work of others, and he expresses anxiety that he might not get to experience all of the great literature he wants to before his death.