It's immediate storage for your computer to quickly access data. For example:
You have a word document saved on your hard drive. When you open it, the data is copied onto the RAM which the rest of the computer uses to interpret it, display it on the screen and allow you to edit it by typing. But this only affects the document in the RAM, not on the hard drive. When you press save, the document on the RAM is used to update the file on the hard drive.
For regular light usage - internet browsing, listening to music, documents and presentations - 4GB is sufficient. But the difference between 4 and 8 GB these days is so small in price, most would opt for 8.
As you want to start doing things like playing games, digital media editing and such like, the minimum RAM you'll need can quickly rise up to 64GB and, recently, even more. Generally, 32GB onwards is usually installed on heavier duty machines for things like 3D animating and music production.
What should I do?