CPU usage is how much processing power you're using up.
For example: If you're playing a video game, and you have a processor with 4 cores, the video game could actively be using two of those cores, and be running at around 50%.
Memory usage refers to your RAM (Random Access Memory) and how much is being used up.
For example: If you have your internet browser open and you have 5 tabs open, each one of those tabs could be taking up (hypothetically) 200mb of RAM each. If I were to assume you had 4GBs of RAM, then you would be taking up 25% of your memory (probably a little more, actually), because you're using 1GB of RAM out of the 4GBs that you have available.
This scenario also describes what's likely happening on your computer. You probably have some low-power programs running like an internet browser open, with a bunch of tabs open. You're not using much processing power because you're not doing anything like processing a video or running a game, but you're taking up your RAM by simply having the window open, since applications use your memory to run and stay open.
In reality, the whole concept of CPU usage and memory usage is a lot more convoluted, but that should give you a general idea of what's happening.