system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 - What is a DJ like Tiesto actually doing when performing a live show?

I don’t really know much about the process of remixing a song but I can imagine it is a process that is like editing a video - layering tracks, adding beats, etc. They are clearly not doing that kind of editing live so they are basically playing something that’s already had some amount of that already done. 

So what are they doing live besides maybe mixing between songs? 

I just don’t understand what all the knob fiddling is about.
gpt
In order to get a basic understanding, imagine a DJ standing in front of actual turntables. They start playing a song, and it has a downbeat (imagine if you're bobbing your head to a groovy song, when you bob down, that's the downbeat). As that song nears it's end, the DJ doesn't want dead air, so they choose a song that has the same beat. They put the record on the other turntable, and listen to it with their headphones. They listen and slow down/speed up the second song until the beat lines up perfectly, then fade the volume of the first song down while fading the second up. That way you're listening to a new song, but the transition was so seamless, your head bobbing hasn't stopped, or even been noticeably altered.

A skilled DJ can fade back and forth between two or more songs, do things like slip the hook from one song into the place where the hook would have been in a different song, and all sorts of other tomfoolery. The more sources of songs, samples, and sounds, the more knobs they need for volume, tempo and effects.

Then imagine someone mixing the sound for a live band. They have a slider for how loud the drums are in the house speakers, how loud the vocals are in the speakers playing back to the band. The DJ needs these too, so they can alter what they are hearing and what the house is hearing separately.