I don't know whether am on the right board but I need some little help to grasp something.
I have been trying to read about the 51% attack but am having a hard time to understand the part where the attacker can "double spend the coins"

So in a layman's language
1.  What is a 51% attack on the blockchain?
2.  What would happen if someone managed to pull this off say on the bitcoin network?

Anybody well informed about this, I will appreciate your time and help.