RAM (Random Access Memory) is a hardware from a computer that works as a temporary memory. It works at much higher speeds and it processes data and programs that are started or that currently are running. RAM is accessed by CPU and other random I/O devices.
All programs and data are executed from RAM, but before executing the data or programs must be fetched from where they are. If there is insufficient free memory then some memory will be freed up by writing some data on the disk. This is called “Caching”. With cache enabled, the server will take longer time to process the operations, and can show errors or lagging.
For a server RAM is one of the main components that is used to run PHP, different code, scripts, plugins, databases and all other applications from your server, all the data is processed through the RAM.
RAM memory is temporary and volatile. RAM keeps data when the system is powered on and running, when server is turned off, RAM doesn’t keep any data.
If a sever will have insufficient RAM to run codes, databases, the website performance and load time will fall drastically and can happen that some services to start often to fail like MySQL.
So when on your server there is insufficient RAM, you can try to enable caching. Caching means that you will create on your disk a virtual RAM image file that will be have the same function as RAM hardware. Unfortunately the server disks don’t have the speed of RAM and will process the data much slower. Anyway this can be considered as a temporary solution.