Things that helped me because setting up a rack is a STEEP learning curve and can take you weeks or even months to fully set up!
decibels, levels, metering, clipping, 0 VU/ VU metering, signal to noise ratio (SNR)
ground loops
balanced connections vs unbalanced, and the effects, advantages of using them
patch bays, splitting, mixing, monitoring
being able to solder your own cables can be extremely helpful bc there will be lots of different size (custom) cable runs
be sure to read this before hooking up your eclipse