A supermarket is like a big store where you can buy lots of different things to eat and drink. It's like a giant pantry that you can walk around in and choose what you want to buy. It has aisles (like roads) with shelves on either side (like houses) that have food and other products on display.
When you go to a supermarket to buy groceries, you will find things like fruits, vegetables, bread, milk, meat, and canned goods. There are also things like candy, chips, and other snacks.
You usually walk around the supermarket with a cart or a basket, which is like a little wagon you can push, to hold your groceries. You can choose the things you want to buy, and then you take them to the checkout (like a big counter) where you pay for them.
The people who work at the supermarket are called employees. They help you find things if you can't, they can also ring up your groceries for you and bag them.
When you leave the supermarket, you take the groceries you bought home and put them away in cabinets, pantry or in the fridge so you can eat them later.