Welcome to Android Central! We need to clarify what type of memory you're talking about. RAM is different from internal storage. RAM is where open apps reside while in use, while internal storage is where the files for those apps are stored. Most phones have 1 or 2 GB of RAM, and at least 8 GB of internal storage.
It is normal for the system to try to fill up RAM with open apps, usually to around 70-80% full. Don't make a habit of clearing RAM--this isn't Windows. Android prefers RAM to be mostly full, and will automatically open apps again if you clear RAM out. This cycle can lead to increased battery drain and inefficient app switching/opening.
Internal storage can run low if you have installed lots of apps or if a lot of data has been generated by those apps. If your phone came with a relatively small amount of internal storage (like 4 or 8 GB), it can get used up pretty quickly, such that clearing app caches frequently won't do much.