Greetings squidling! You, indeed, asked in the right sub-forum =p
The short answer is: callouts are team/squad voice communication
-things like "e-liter is splatted" or "kraken on the (splat) zone"
The long answer?
I don't want type that out at the moment, but thankfully inkipedia is here to save the day!
http://splatoonwiki.org/wiki/Callouts
There is soooo much information here it hurts (in a good way?)