Imagine you have a toy store, but instead of selling toys to kids, you only sell toys to people who work for your dad's company. That is what a company store is - a store for employees of a particular company.
For example, if your dad works at a big car company, the company might have a store where only workers can shop. The store sells things that the workers might need, like uniforms or safety gear.
The company store might also sell snacks or drinks, like a vending machine. Maybe the workers can buy t-shirts, hats, or other merchandise with the company logo on it.
Sometimes, the company might give the workers a special discount at the store. This makes it easier and cheaper for them to get things they might need for work.
So, a company store is a special store only for people who work at a particular company. They sell things the workers might need and sometimes offer special discounts.