Overclocking simply means increasing the clock frequency of a certain component (CPU, GPU, RAM) beyond what the manufacturer originally set it to. So if your FX 8320 is clocked at 3.5 GHz out of the box, increasing the frequency to, say, 4 GHz would be what overclocking means. Since these components can only perform a certain number of operations per clock cycle, overclocking generally increases the performance of a component.
There are many tutorials out there about how to overclock.
Like this one. Or this one.
The basic concept is you go into your BIOS and increase clock speed incrementally until you see instability. Then you increase voltage to stabilize the overclock and go back to increasing clock speed. Repeat until you are at a clock speed/voltage you are comfortable with.
Overclocking (especially overvolting) generates a lot of extra heat. So if you are going to overclock a CPU, you will need to get an aftermarket cooler for your CPU. Stock coolers aren't going to cut it. Good aftermarket coolers include Cooler Master Hyper 212 Evo and Noctua DH14.
Overclocking GPUs is generally easier than overclocking CPUs because there are plenty of third-party apps (EVGA Precision X and MSI Afterburner) that allow you to do it without having to go into BIOS. But the principles are the same: increase clock speed until you see instability, at which point you will need to increase voltage to go further, making sure always to watch for overheating.