In addition to its normal meaning, “to dry”, sécher has two very different colloquial meanings.
The first one is “to have no idea about the answer” of a question or a puzzle.
Dis-moi la solution, parce que je sèche complètement.
Tell me the answer, because I really have no idea.
The second one is “to skip class”, “to play truant”.
J’ai séché le cours de français ce matin.
I skipped the French lesson this morning.
Il a été viré de l’école parce qu’il séchait trop souvent.
He was expelled from the school because he skipped too much classes.
Listen to the examples: