The basic backbone of the computer is a processor (CPU or central processing unit), which is responsible for all the calculations that run at the back-end which result in whatever that you see on the computer screen. The processor has a limit of how much calculations it can do at one time. Every application you run (some you run yourself, some are always running to support the operating system, some are ideal and only run automatically when the cpu is free) require some sort of calculations in order to run..so the more applications you run at one time, it means the calculations that are taking place at one time start to increase, so does the processing power required. The cpu meter just shows what percentage of processing power is being currently utilized in contrast to the total processing power. If you open up a lot of applications and your processor isn't very powerful you may experience a bottleneck with the processor, that is when the programs start to lag and you restart your computer or close down lots of programs for it to return to normal speed. (Although there are other reasons for lag, this is one of them.)