A supply of something is an amount of it that you have that you can use. For example, you probably have a supply of food in your pantry and fridge. And, hopefully, you have a good supply of pencils and notebook paper for school.
Those items, like food in your pantry and notebook paper in your backpack, are your supplies. Supplies are things you need that are useful. In other words, supplies are items that help you do what you need to do. You use them up, and then you need to get more.
For example, school supplies are things like notebooks, pencils, and folders. You need these supplies to do your schoolwork. As the year goes on, and as you use them up and wear them out, you'll need to buy more supplies.