Here's the relevant passage:
> "When I have fears that I may cease to be
> Before my pen has glean'd my teeming brain,
> Before high-piled books, in charactery,
> Hold like rich garners the full ripen'd grain;"
The speaker fears dying before they can fully explore their creative potential. They see their mind as "teeming" with ideas, and the books they hope to write as "garners" full of "ripen'd grain." This grain, the knowledge and experience contained in books, is compared to the vastness of the night sky, implying an endless, and potentially unattainable, amount of knowledge and experience.
The speaker is essentially grappling with the overwhelming amount of experience and potential within their own mind and the fear that they may not be able to fully express it.