For just three controls to be the most important out of thousands must make them pretty special, and they are. They are among the console status controls and are labeled RECORD, REPLAY and MIX. There could have been another one called OVERDUB, but you can select this mode by pressing the record and mix buttons simultaneously.
These four button combinations neatly summarize the basics of multitrack recording...
In your flight of fancy where you are the engineer operating an SSL mixing console, imagine also a band in the studio. (Naturally the console is in the separate control room).
Starting off with a blank tape, the first thing you will want to do is record 'basic tracks' where the band all plays together.
Each channel module has two faders - the small fader is used to control the level going to the multitrack recorder. Clearly the level must not be too high as to cause distortion; also it must not be too low otherwise there will be more noise than necessary. The large faders of the channels are used to create a 'monitor mix', which doesn't affect the levels going to the multitrack, but is purely so you can hear the music in a reasonably correct balance as you record.
After recording, the next step is to play the take back and assess its merits. It is convenient if there are loudspeakers in the studio so the band can hear the playback without coming through to the control room.
The third stage is overdubbing, where additional instruments or vocals are recorded, in sync with the tracks that have already been recorded.
The final stage is mixing, where all of the instruments and vocals are blended to the correct proportions, creating the mix that the listener will eventually hear.
For more detail on the Record, Replay, Overdub and Mix modes of the SSL mixing console, read on...
Record: used when recording basic tracks
Replay: used for playback of basic tracks
Mix: used for mixing to stereo
Record + Mix (Overdub): used when adding overdubs to basic tracks