Scales are small plates that grow out of the skin of fish and reptiles. They overlap, so if you run your hands down a fish or snake from its head to its tail it will feel smooth, and if you run your hand the other way it will feel rough.
Scales are a device used for measuring how heavy something is. There are many different types of scales. Originally scales were made to balance like a seesaw, and you put the thing you wanted weighed on one side and weights on the other side until it balanced out. Today, scales are often digital and need batteries to work. There are scales you stand on so you know how much you weigh, kitchen scales like in the picture so you can weigh food, and really big industrial scales to weigh trucks full of grain so it can be sold by weight.