Linux has this basic rule: a page of free RAM is wasted RAM. RAM is used for a lot more than just user application data. It also stores data for the kernel itself and, most importantly, can mirror data stored on the disk for super-fast access, this is reported usually as “buffers/cache”, “disk cache” or “cached” by top. Cached memory is essentially free, in that it can be replaced quickly if a running (or newly starting) program needs the memory.
Keeping the cache means that if something needs the same data again, there’s a good chance it will still be in the cache in memory.
So as first thing in your system you can use the command free to get a first idea of how is going the use of your RAM.
This is the output on my old laptop with Xubuntu:
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