51% attacks , does this mean they need to have 51% of the excisting coins.. to do that ?
No. It means they need to have 51% of the computing power used of mining.
I'm not sure how much of the technology behind Bitcoin you understand, so I'll try to explain it simply. The big feature of Bitcoin is that when you send someone your coins, the transfer is unambiguous and impossible to undo. But the reason that works is because of Bitcoin miners, who make their computers do busywork in order to "vote" on the order in which they saw transactions occur. Thus, in order to fork the blockchain, or undo a transaction, or stuff like that, an attacker would need enough mining power (computers doing busywork) to outvote everyone else. Under normal circumstances, it's reasonable to assume that this would be infeasibly hard.
But if you have an attacker who is willing and able to sink huge amounts of money into an ASIC-based supercomputer before the honest miners have adopted ASICs themselves, they could break that assumption.
