Rules can set you free. Sometimes writing about anything can be very difficult. You have anything to chose from after all. But if you narrow the starting parameters it can force you into a subject matter. You will also make choices that you maybe wouldn't without the constraints.
So if I asked you to write about anything you may struggle. But if I said write a ten line poem that has five words in each line and is about cactuses, with the same line to start and end, you might write something more specific and more surprising.
Constrained writing is therefore any writing that follows a set of rules.
The Oulipo movement in the 60's was a big fan of rules.
Poetic form is basically a set of rules that constrain your writing.