Obligation: should, ought to, must.
Should and ought to express suggestions, advice.
They tell you, what in the speaker's opinion, is the best or right thing to do.
For example:
You're always broke, you should try spending less money on clothes.
You ought to be more careful with money.
Must expresses strong obligation.
You must go visit your grandmother, she's sick!
Everyone must show their ID cards to get into a club.
Books must be returned to the library before the due date.