This idiom or expression is about:

Supporting someone.
Believing in someone.

I’m tired of being a quiet observer. Starting today, I’m standing up for victims of violence and their rights!

Also, see:
Taking a stand.

This idiom or expression means:

Answering questions truthfully.
Going to the witness stand (in a court of law) and answering questions under oath.

As a witness you’ll have to take the stand and answer questions truthfully! Are you ready to do that?

This idiom or expression means:

Choosing a cause.
Supporting someone or something.

Also, see:
Standing up for someone.