"Give (someone) a hand" means to help someone. The word "help" has a wide meaning. It can be used to describe helping someone with a small problem, or with a serious emergency. So when you're asking for help, it's important to express how serious the problem is. For big emergencies, you just yell:

Help!

For smaller but still serious problems, you can ask:

Can you help me?

And for small problems that aren't an emergency, you ask:

Can you give me a hand?