A cookie is a small piece of data a website can store on your computer, and it will be included with every request your browser makes to that website. While most visible cookie usage (I.e. the endless popups) is just to track you and violate your privacy, their are plenty of legitimate uses for cookies:

• store settings you make on a website, such as dark mode or (ironically) that you don’t want to be tracked
• store a session ID so the website can store more data on their end, such as the contents of a shopping cart. This is also often used for analytics, which is basically following the paths users take through a website to see if everything works as intended.
• store a successful login to a website. Most websites will require a cookie to allow you to log in. There are other ways to accomplish this but they’re ancient and annoying.

Basically you should keep declining cookie popups, except for functional cookies on a website where you actually want to do stuff (login, shop, etc) instead of just reading something.