blah blah blah
Linux is borrowing unused memory for disk caching. This makes it looks like you are low on memory, but you are not! Everything is fine!
If your applications want more memory, they just take back a chunk that the disk cache borrowed. Disk cache can always be given back to applications immediately! You are not low on ram!
The above is very similar to how iOS handles cache. As I said, research cache and flash memory and how they work together. You cannot learn unless you look it up. Being incurious doesn't change the facts.