Welcome to the forum.
Loop on means just that, the FX Loop is always on regardless of amp channel.
CH1 has the FX Loop on for CH1 only. The FX Loop will be off in the other channels.
CH2 works the same as above, but the FX Loop is on on for CH2
CH3 follows above logic.
FTSW means the FX Loop is switched on/off exclusively by the button on the Footswitch Controller.
Bypass Loop will completely bypass the FX Loop and all of its associated circuits. The Output and Solo controls will also be bypassed.
Your amp has a parallel FX Loop. There are pros and cons to this configuration, but it limits what effects you can utilize. It is designed to mix in the wet signal of the FX Loop with the dry signal of the amp using the Mix control. Regardless of the 100% label on the mix knob the parallel FX Loop
will always have a dry signal path that will bypass the FX Loop making nose gates, volume pedals and EQ’s pretty much useless in the FX Loop. I prefer (& most Recto players as well) a series FX Loop, there is no separate dry signal path, the FX Loop is directly in the only signal path. The amp can be modified to a series FX Loop.
I recommend you have a look through the user manual for your amp, this and all the other features of your amp are explained in detail.
I believe this would be the correct manual:
https://mesa-boogie.imgix.net/media/User Manuals/3chRecto.pdf
Dom