Some reverberation units have indipendent settings for dry signal and wet signal, so you can mix them separately. The mix control allows you to blend the sounds whit a single parameter. The same difference between volume sliders in a dj mixer and the crossfade.
And "send" means send
You simply send the signal to the inputs of the reverb, and with the "send" control you adjust the amount (in term of level I think) of the sent signal.