Memory

Memory is what your computer uses to load the current applications into when you work with it.

If there isn't enough memory to load everything at once then the computer will 'page' memory to the long term hard drive storage which is slower. This is somethings called memory thrashing.

Think of it like yourself if you have too many tasks at once. It's hard to concentrate of them all at once.

Background tasks can compete for memory

If you have installed software which runs in the background then this can compete for memory (and other resources like the the CPU.

Rebooting your computer frees memory

If you do not periodically reboot your computer some processes will take memory and not give it back.

This is called a memory leak. Rebooting will help.