This mainly means that you raise the default speed of your components. So if your CPU comes at 2.9ghz, you later run it at 3.5 (just an example).
This can be done easily nowadays but used to be a big task in earlier times.

There are various guides around the net, so if you have a running system you may want to read and try.
If you stay within save limits of temperatures and voltages, no harm is done but as the components only guarantee you their default speed, you may encounter instabilities when aiming too high or approaching somehow wrongly.

To get to know those limits is the task then. But, as said, if you google "Athlon X2 overclocking" or something you'll get more than enough information while your first task now is to complete your system and run it.  ;)

For the overclocking thing, a fellow pc enthusiast sitting in front of your PC could be more helpful than any guide on the net.
As FSX generally is CPU limited, the speed of your CPU therefor is the value to raise while your graphics card comes in second place and usually won't give you big gains in performance.