Memory (DRAM) is a component in your computer that allows your system to perform many of its everyday tasks, such as load applications, browse the Web, edit a spreadsheet in Excel, or experience the realism of the latest game. Memory is what allows your computer to perform its basic functions. As a general rule, the more memory you have, the better.
If you turn on your computer, load Excel, and begin working on a spreadsheet, you'll have just used memory in several different ways. Whether it's loading and running applications, responding to commands, or toggling between multiple programs at once, memory is almost always being actively used by your computer.
Memory is your system's short term data memory (it stores the information your computer is actively using). The more programs your system is running, they more memory you'll likely need.
In a way, memory is like your desk. It allows you the space to work on all sorts of projects, and the larger your desk, the more papers, folders, and tasks you can have out at once. If you leave your desk and want to put your projects away, you might decide to put them in a filing cabinet for safekeeping (your storage drive).
If your system is slow or unresponsive, a memory upgrade is one of the easiest and most cost-effective ways to improve performance.