A double negative is two negatively expressed ideas (either grammatical or semantic) in a sentence, which cancel each other out to become a positive e.g.
I don't like not knowing = I like knowing
She isn't unattractive = she's attractive
I haven't found nothing = I have found something
I dislike nobody = I (probably) actively like some people
These are just a few simple examples...double negatives can express many nuances of meaning, but hope this helps!
Student or Learner