‘Doesn’t let’ sounds more general. You would use it for a general rule. “My boss doesn’t let me wear earnings”. But ‘won’t let’ can also be used for things that only happened once. For example, if your dad told you that you couldn’t go to a party today because you have to do you homework, then you would say ‘my dad won’t let me go to the party’. If your dad never let you go to parties then you would say ‘my dad doesn’t let me go to parties’. You can also say ‘my dad won’t let me go to parties’ and that has the same meaning.

Give examples. These words can be used in an enormous variety of situations. Examples can pinpoint what it is that you don't understand.