What happens if someone knows your password?

What happens depends on who it is that knows your passWord and what the passWord protects.

Worst case is terrible - a nefarious person could do anything that you can do at your computer- transfer money, make charges on your credit card, copy , alter or delete important and sensitive data, etc, etc.

Of course a trusted friend who knows your passWord will not do you any harm, except maybe by accident.