no such thing as a stupid question
a HOB Overflow has two 'boxes' one inside the tank with little teeth to do some surface skimming, and one on the outside w/ two chambers. The first chamber on the outside stays full of water so your siphon doesn't break. The siphon is a 'U-tube' which sucks the water out of the display tank, dumps it into the first chamber of the outside box on the overflow, which in turn dumps it into the second chamber, which has a drain that drains the water down into your sump / refuge area.
It sounds really complicated, and I myself was baffled at how they worked, until I set it up. After I set it up I was all like 'whoa, that makes sense and is simple'. :yes:
Hope that didn't confuse you more.