This is a very casual, often regional way to say one when you mean a person or a thing of a certain kind: a good 'un, a bad 'un, their young 'un. It always sits after a describing word. Teachers and careful speakers do not use it. You will see it in stories that try to catch how someone from a place really talks. Though actually, the apostrophe is a bit of a fib. This is not one with a letter taken off. Old English said one without a w sound at the front, more like “un”, and this spelling keeps that old sound. So it looks like a short one, but it is really an old pronunciation wearing an apostrophe.