The key thing is: if everything's working as it was, then you've probably done no harm. BIOS = basic input/output system and, at the lowest level, it tells the computer what it is, where everything is and what to do before trying to load e.g. Windows. You can make changes in the BIOS, such as the order in which you want it to look at devices to boot from (e.g. if you want it to look for an operating system on the optical drive before it looks on the hard drive). It's also where you would change important things about its operating characteristics, such as uprating the processor speed ("overclocking"). All you've done by resetting the BIOS is to return your PC to how it was when it left the factory. Yes, if it was overclocked, then it will likely not be overclocked any more - but you may well not notice any difference.
Xiaomi Mi Mix 2 review