Pull-on clothes or shoes have no buttons, zips, or laces you have to do up. You just pull them onto your body and they stay. Shops talk about pull-on boots, pull-on trousers, a pull-on hat. It is mostly an adjective in clothing descriptions, sitting in front of the noun. The idea is ease: you get dressed in one movement. Slip-on is a close cousin, especially for shoes, though slip-on often means you slide your foot in without even a tight pull. You can also pull on a jumper as a verb (no hyphen needed in the same way); the hyphenated pull-on names the kind of garment. It is practical shop language, not poetic.