Basically overclocking is making your CPU run faster for a "boosted" performance. K - series processors have the ability to be overclocked out of their manufacturer set frequency. In this case, 4.0 GHZ. Say you overclock it to 4.5 GHZ, you get a slightly boosted performance than non-overclocked 4.0 GHZ i7-6700k would originally give.

However, If you're not comfortable messing with all that I would just let it be.
That motherboard though should have some sort of auto-overclock button/function added onto it though. That ASUS would deem safe/doable on most/all CPUs.

https://uk.pcpartpicker.com/product/...-cpu-cooler-h7

I personally have this CPU cooler, was a little hard to put on myself but was manageable. Also was able to overclock my i5-6600k from 3.5 GHZ to 4.3 GHZ, but I found that some high rendering processes were a little bit unstable so I put it down to 4.1 GHZ where it's very stable.