Memory of RAM is the "thinking space" or "short term memory" for your computer.
Think of it like a kitchen worktop... the more worktop you have, the more you can do.
Memory is measured in Gigabytes (abbreviated to GB).
The minimum you should have in your computer is 2 Gigabytes.
The recommended amount of RAM is 4 GB.
If you like to do lots of things with graphics or run several programs at once, then 4 GB is adviseable.
If you need a very powerful computer, you may need more memory.
There are a limited number of slots inside your computer to fit new memory, and there is a maximum amount of memory which each computer can have.
There are many different types of memory (e.g. DDR2, DDR3, non-ECC, ECC) - always get advice before buying memory.
For reasons of space, laptops and tablets are often limited to the amount of RAM they can hold.
Tablet computers are often sealed units and it might not be possible to open it to increase the RAM.