The cuff of a pair of pants refers to the two holes at the bottom of the pants where the feet come out, and the surrounding fabric.
In general, the diameter of the cuff is pretty consistent with the diameter of the rest of the leg. However, with some styles of pants, such as bootcut jeans and bellbottom jeans, the cuff is much wider than the rest of the leg, which gives the wearer a wider base.
In general, the cuff of your pants (if you’re wearing long legged pants) should come down to right above your shoes or slightly over it, although slightly higher can be appropriate as well, especially if you’re wearing some nice socks you want to show off.
Many guys wear their cuffs rolled up, either out of practicality or as a fashion statement. Some pairs of pants are double-sided, so rolling up the cuffs reveals the inner designs. Rolling up the cuffs of jeans has also recently become fashionable, largely in part to wearers of selvage denim who roll up their cuffs to show off the self-edge of their jeans.