What is a cookie?
A cookie is simply a file that is deposited on your computer by a website you visit. E.g., if you visit the website for the Avengers movie, the site will create a file on your computer that it can lookup next time you visit the same site.
What is the big deal?
If there is a cookie on your computer, the site will know that you are a returning visitor and perhaps say “Welcome back!”. It will know what links you have already visited and use this to show you new links. This way it will personalize your experience.
That sounds wonderful! Are there things I should be worried about?
Sometimes the way the site transmits its cookies to your computer and the actual content of the cookies could be intercepted by hackers and that could violate your privacy. They can also be a security risk on some sites. For this reason you should accept cookies only from sites you trust.
Cookies are deposited not only by sites you visit but also by ads you click on. Example: if you visit a website for a Harry Potter movie and you see an ad for Kit Kats, there might be a Kit Kat related cookie deposited on your site if you click on it. This information is how they know to retarget you across different websites.
Sounds like I should not accept cookies at all to be safe?
You can do that but then some websites will just refuse to serve content if you do not accept their cookies. So there is a balance between personalization and privacy you will have to strike.