The meaning is:

I got a 20 on my homework/assignment. (i.e. I got an A+)

Traditional homework/assignment rating in French schools is based on a score ranging from 0 to 20 so this is (in theory¹) the best possible score.

In France, this grading is the official one from the middle school up to the universities and grandes écoles. Primary schools might use a grading from 0 to 10, letters A to E, or an acquisition status: A, ECA, AR, NA. See https://fr.wikipedia.org/wiki/Note_scolaire for more details.

¹_{In theory because there are teachers that never give a 20, especially in non scientific subjects and those not based on MCQs, but it used to be more the case in the past. On the other hand, sometimes the compound grade might be higher than 20 because extra points can be earned with optional questions / subjects.
See https://www.orientation-education.com/article/21-33-de-moyenne-au-bac-un-record}

As Jlliagre said, Tests in France are often graded on 20. So “20” is short for “20/20” (“vingt sur vingt”).

I’ll add that “devoirs” (plural) is homework, but “un devoir” is a test or a graded assignment, often called “devoir maison” (or “DM”) when it’s to be done at home.

The singular “devoir” is almost never used for ungraded homework anymore.

Note: this answer is only about France French. Vocabulary may vary in other countries.