"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;
When I behold, upon the night's starr'd face,
Huge cloudy symbols of a high romance;"
This sestet is the last six lines of the sonnet.
Here's why it's a sestet:
* It consists of six lines.
* It's part of a larger poem, a sonnet in this case.
* It typically has a distinct shift in tone or focus from the preceding quatrain. In Keats's sonnet, the sestet moves from a fear of premature death to a celebration of the power of imagination.
Let me know if you'd like more examples or want to explore other poetic forms!