The RAM or Random Access Memory is the work memory, where all the programs and data information that are open in a defined moment is loaded. The RAM only stores data information when the computer is on, when we computer is off the information on the RAM is cleared.
When a user turn on the computer, the system read the BIOS, the BIOS is a integrated circuit that store the basic computer configuration, for example the CPU speed the numbers and size of the hard drives, the integrated ports, the boot secuence, the date and time, the RAM available, etc. After the system read and check all the devices and settings, the system check the default boot device, this device commonly is the hard drive HD or the DVD-ROM, then in this way the device boot sector is readed and the boot secuence is initiated loading all the instructions on the RAM with the help of the CPU. When the system operative is loaded, for example when the desktop is showed to the user, many files and process are working and loaded on the RAM, and when you are just moving the mouse cursor, many things are happen on the RAM, in the same way, when you open a program like Microsoft Word for example, many files are readed and processed until the program is completed and loaded in the RAM, and is waiting for your interaction, also in this moment all the MS Word files needed to work are loaded in the RAM and only when you close the application this files are cleaned, and the memory space recovered. That is the importance of have high amount of RAM on a computer, if you have many RAM, you can load more process, data and applications, and your computer is faster, but if you have low RAM, the system cannot find a work place to load all the needed information, and try to emulate RAM with other resources like the hard drive (if the hard drive have space), or in other scenarios just blocking the computer and working in a slow way.