Memory is what the Android device uses to manage currently running applications.
Note: Not to be confused with internal-storage.
Users new to Android (and not familiar with Linux either) often wonder why the RAM is always "full" -- or, put in other words, why there is so few "unused RAM". Before asking a question in this direction, you should get a little familiar with how RAM is used on Linux in general and Android in special: free RAM is wasted RAM1.
There are a few questions here on our site which can serve as good starters, as they have answers explaining this more detailed:
Please make sure to have understood at least the basics before you ask your question. Might even be, after reading the above your question is already answered.