What is cache RAM?

Hi all. The question is, is there simple server for 2 GB of operatives with one core, there is a few new sites on WP, and so often include htop and leave in the background, RAM always takes no more than 1 GB

but in the West that such a graph always

ie talking to other people, I realized what htop shows in figures the value of memory which uses the system now, and in the West with cache use, you may not know this)
After that, read what is memory cache I have presented it, namely, I understood that the system by default takes up all memory (almost) which is free in the cache and then when a new process starts it gives it from cache, it's faster.

Making conclusions that in this case I have almost 1 GB of RAM (in the cache),

I started on the server a simple parser that the LAN takes very little memory, and mysql blacked out almost immediately with the start of the parser, apparently she didn't have enough operatives (before it was turned off because of this) but why?, turns out I misunderstood what a cache operatives?, or still need to dig into the side of the parser and I correctly understood?

PS. I ask not to throw stones for not very smart question), I understand a little, and I would like more)