Cookies are short pieces of information that a web page can store in the browser for later use. Many people confuse cookies with cache, but they are something slightly different.
Use
Take, for example, the shopping cart in an online shop. You add goods to the cart and this is saved as a cookie, that is, information about adding one kind of item to the cart. You then continue browsing the shop and add another. All the data is processed only when the order is submitted. Another use is logging in. If cookies did not exist, you would have to log in again and again on every page of the same website. When you log in, the cookie simply stores that you are logged in, and this is what the site works with from then on.
This raises the question of security. The information in cookies can be read only by the websites that created them. Other websites may, for example, use cookies only over an encrypted connection, and so on.
It used to be said that using cookies was dangerous. Today, however, cookies are on practically every website.