[Sir Nathan XXI]

predrilled means the tank has holes drilled in the glass, usually the bottom, then a wall like a dam is build around that to maintain tank water height, one hole if for a drain the other is for your water return line, this setup is to connect to sump filters,

here is a pic of it

pic is from all-glass.com

 

[Sir Nathan XXI]

What happens is this

power off on pump: you fill tank to its full hieght, then you fill sump up a little past top of pump

turn pump on: water is poured into the tank via pump from sump, water then overflows into the dam section and is drained into the sump,

so basically its a continous trading of water back and forth, but continuous

a sump is a holding chamber, basically another tank, where people put heaters, skimmers, wet-drys, chemcial filtration, auto-water changers, etc etc