What does the "nos gusta" clarifier do? Doesn't nosotros already just mean we? What does it clarify?

Gustar is a reflexive verb, so for reflexive verbs you have to put a special article before the verb.

For example, 'me' gusta or 'te' gusta. If gustar was a regular verb you would say yo gusta, or ellos gusta.

The reflexive article for nosotros is nos, so when you say "we like", it would translate to "nos gusta".