Poder 

The verb "poder" is very common in Spanish and is used to express ability, permission, or possibility. Here is a simple explanation:

Capacity: Used when someone has the ability or strength to do something.

I can run fast (I have the ability to run fast).

Permission: Used to ask for or give permission.

Can I go to the bathroom? (You ask for permission to go to the bathroom).

Possibility: Used to express something that is possible or that could happen.

It may rain this afternoon (It is possible that it will rain).