An over flow is generaly used in conjunction with an external tank or filtration "system...such as a sump or refugium or wet/dry or any combination or series of said units.
These external tanks generaly have a return pump in them to move water into your main tank.
If these return pumps continue to add water to your tank and it has no where to go it will eventually "overflow"...over the top, the sides, the back and down to the floor, into the carpet, furniture and eventually the basement.
So, the "overflow" is a device to direct this extra water back to the external tank.
They generaly come in two types with many variations...ie:internal or drilled, or external or over the top/hang on back.
The internal or drilled system generaly consists of a hole drilled in the botom of the tank with a waterproof 'bulkhead" installed and a riser tube to set water depth...or it may have a built in "corner box"
installed in the tank to set water depth and the water returns through a 'stockman" or "Durso' pipe or may just drain through the bulkhead and return line.
The external or over the top system consists of two "boxes" one inside one outside the tank, with a "U" tube connecting the two to create and maintain a siphon effect...moving the water from inside the tank to the outside box where it can then drain down to the external tank/system through the above named pipes and drain lines.
Don't you just hate overly complex answers to a simple question???