adding to above...
1) this means you can EQ or effect each sound differently.
common example is drum machine, which generates
both a kick & snare, you might want kick distorted with a little
EQ & no reverb, and snare clean with different EQ & some
reverb. you can't do this if they are both coming out
of the same output on the drum machine; they need to
be separated. With a multitimbral box, you can't even
adjust the relative levels of say a pad & lead sometimes
without having them split out...most let you do this much
in the box, but.....
2) main inserts are just like channel inserts, but for the whole
mix. so i'm guessing you don't understand a channel insert?
channel insert = TRS cable broken out, tip = send,
ring = return. Signal goes through pad/gain, then through
EQ, then out the insert via the send. This is supposed to
go to an effects unit, compressor, whatever. The output
of this effects unit, whatever should be hooked up to the
ring so it can be returned to the mixer. A loop. Thus
called "Insert". you need a cable with a stereo TRS
on one end and 2 mono TS jacks on the other, the
tip of the TRS is connected to the tip of one TS jack
and the ring of the TRS is connected to the tip of
the other TRS jack is you make these yourself.
the master inserts also let you do this, but after all the channels
have been mixed, and usually before the final fader.
3) yes, hook them series. send -> FX A -> FX B -> return.
no problem. you can do this with any insert.