The selvedge is the special edge of a piece of woven cloth, made strong so the threads will not fray and come apart. It is not a hem you sew later; it is built in as the cloth is made, because the crosswise thread turns at the side and comes back. You will also see the spelling selvage, especially in American English — same thing. People who love jeans talk about selvedge denim: you can often see a tidy, sometimes coloured line along the outseam, which is that factory edge left on purpose. A student who only knows “edge” might think any cut side is a selvedge; it is only that finished woven side.