This idiom or expression is about:

Supporting, believing in someone or something.
Defending or protecting someone, their ideas, their honor, their physical well-being, etc.

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 has to do with:

Seriously believing in something.

When someone says “I swear by warm, steamy baths to relax the body,” they mean something like “I seriously believe that a warm bath relaxes the body!!”

This idiom or expression has to do with:

Believing or accepting something.

When someone says “No one’s buying your story,” they mean “Nobody’s believing you.”

Also:
Buying it.
Buying into something.

This idiom or expression has to do with:

Believing something, as in:

A. You think she hates me?
B. Yes, count on it!

Relying on someone or something, as in:

Listen guys. I’m gonna start campaigning for better wages, but I can’t do it alone. Can I count on your help?

This idiom or expression means:

Believing.

I took stock in his proposal and supported it, but I wish I hadn’t!