Changeable or detachable refers to the sole of the shoe. It means the cleats or spikes can be detached and changed when the football cleats are worn down, so you don't need to buy a complete new pair of football shoes. Plus detachable cleats allow you to have more control in terms of customizing the studs for a specific type of playing surface. Changeable football cleats ar often a bit more expensive than moulded football shoes, so unless your feet have stopped growing it might be a cheaper option to buy moulded football shoes.
Also many (youth) football organizations require only moulded cleats, and won’t permit detachable cleats being used. So check with your coach before buying