RAM is Random Access Memory, it is where the computer processor stores temporary or high frequency data for quick access. It is not stored in any order and is rapidly freed and re-used.
The more RAM you have, the less work your processor needs to do, freeing up used memory with the new data, it is rare to have a single process using over 1GB
a GB(gigabyte) is one billion bytes, a byte is 8 bits, a bit is either a 0 or a 1.
a 32bit operating system supports upto 3.2gb of ram, you can have 4gb, but your pc will only see 3.2gb of it.
If you wish to have more RAM you will need an 64bit operating system and compatible processor.