It means exactly what it says. You have a flap with Velcro on it. You fold it over and attach it to the other Velcro. Velcro = 魔术贴

I have a pair of pants and the two pockets in the back have a piece of material that can be "folded over" the opening of the pocket. Some pockets can be closed to make sure that nothing falls out of the pocket. Some pockets can be closed with a zipper, a button or a piece of the material (cloth) that has velcro. This expression you wrote could be written this way: "fold-over flap" that uses velcro to close the pocket (= with velcro). So "fold-over" is an adjective that describes the kind of "flap" and "with velcro" tells us that the flap will close the pocket by using velcro instead of a button or zipper.

Hi " fold up" usually refers to reducing the size of something, say a piece of paper to fit an envelope or even a piece of equiment such as folding pram to get it into a car. folding over means that something like a piece of paper, or cloth has been folded on one side, either to make a hem, or to reduce the size of it. Hope this helps :)

Velcro consists of two strips or sides, one with tiny hooks and the other with tiny loops. When placed together, this creates a bond. The strength of that bond varies by the strength of the Velcro. In your example you have to sides to join together.