In your System Dashboard, you will see a section about your Cached Memory and your Real Memory Usage. It gives you a percentage of how much memory is currently being used. It looks like this:
Having a high Cached Memory Usage is a good thing and the higher the percentage the better. It means your system is using this space to store some types of processes. Without this being cached then the system would have to create new processes frequently.
Your Real Memory Usage is the actual amount of RAM usage. This should ideally be low. Having a high percentage of real memory usage is normally cause to investigate an issue.