When you go into a store, you take a "shopping cart", which is a big basket on wheels. You push your shopping cart around the store and put things in it that you want to buy.

Websites also use the idea of the "shopping cart" for customers buying things online. If you see something that you want to buy on a shopping website, there's usually a button that you can push which is labeled "Add to cart".

The meaning of "Add to Cart" is "Please add this item to my shopping cart." Websites use the shorter version so that it will fit on a button.