You can read in simple english what RAM is here.
Random-access memory - Simple English Wikipedia, the free encyclopedia
But i guess you already tried that so i am going to make it even more simple. RAM stores data and information about the running programs. The more RAM you have, the faster your laptop is going to be. 8gb of DDR3 (which i assume is the one you're getting as this is the most common one) should be good enough unless you are going to run a lot of programs at the same time or you're going to use resource-heavy programs i.e. video editing, high end games, etc.
If you need me to explain more stuff then feel free to reply here or pm me.