A poetic form just refers to a type of poem that follows a particular set of rules, whether it be the number of lines, the length or number of stanzas, rhyme scheme, subject matter, or really whatever rule you can think of.
The most famous poetic form of all has to be the sonnet. But that's not to say that there aren't a few other ones that are equally awesome:
And we've only just begun. There are forms upon forms upon forms upon forms. And each one has its own variants and spin-offs.