What does Guernica depict?

It does not 'depict' in the normal sense of the Word.

It is an expression of Picasso's Horror when he heard about the bombing of Guernica, Spain, by twenty-eight German bombers, on April 26, 1937 during the Spanish Civil War.

Guernica is a city in the Basque region of Spain. If you google the name, Wikipedia is sure to tell you the whole story.