There are already 2 holed drilled in the corner or middle back of the tank glass for a return and drain plumbing. Most "reef ready" termed tanks come with plumbing too... if not, you have to factor purchasing the plastic/plumbing into your tank costs. For instance, the oceanic "reef ready" tanks just come with two holes... but you can get their "reef ready" plumbing kits that include
1 The big black plastic overflow portion that sections off your tank water from your overflow.
2 The return pipe that is shaped like an upside down hook... this basically lets the water in the overflow chamber go down into your sump/fuge/filtration system below
3 All bulkheads that cover your pipes/tubing from the edges of the glass and make a tight seal at the holes
4 The return pipe with the bendable nozzle to shoot back into the tank.
If you dont wish or cant drill a tank, you will have to use the hang on back filters/overflow that basically sucks the stuff via cyphon up over the lip of the tank and it can cause spills while away if you lose cyphon and/or power. That's why people recommend getting it reef ready or having it drilled if it isn't and purchase the plumbing kits and make it like it came reef ready
I will attach a few little pics that may explain all of this. Hope this helps!