Where are all the options?
It means exactly what it says...to write about what you know. If you know about something than you'll have more to say and have an opinion. If you don't know about a subject...there's less to say.
a poem that does not have a set rhyme scheme or set meter
A free verse poem is just that, a poem that is free. It is free of all rules or constraints. It does not have a set rhyme scheme or set meter. Rhyme scheme is the pattern in which rhymes line. For example, Shakespeare's sonnets have a particular rhyme scheme labeled ABAB CDCD EFEF GG. Meter is the pattern of stressed and unstressed syllables in a poem that creates a rhythm.
Blank verse is a poem that has a specific meter but no rhyme scheme.