be behind (someone) on (something)
This means to agree with someone and support their plan or decision.
When you tell someone that you are "behind" them, it means that you agree with their opinions but doesn't necessarily mean that you will use your time or money to help them.
You don't have to include "on ___" with this phrase if it's obvious. If someone tells you their plan and you agree with it, you can say:
I'm totally behind you.