From his description it sounds like he is talking about the amount of free memory gradually goes down over time.
This could be caused by a number of things...
1) If you open programs you obviously will have less free RAM, but I'm going to assume he meant the amount of free RAM is less when nothing is running. Although its hard to tell what may be left running in the background and using memory, which brings us to the next point.
2) You could have some sort of virus/malware/etc. sapping your resources, try running the normal cleaning tools like malwarebites etc.
3) One of the programs you use may be leaking memory. If it is happening at a startling rate, that is, a gig or more in a few hours as it sounds like you are describing (requiring multiple reboots a day), then you should probably do some google searches on the programs you run to see if one of them is a known culprit.