What is a cookie?
A cookie is a text file that is saved on the client, in other words the web browser. Cookies are used to enable statistics of the usage of a website. In some cases, it can be used to track how the user uses the website. There are temporary and permanent cookies. The temporary variant expires by itself using TTL (time to live), a setting used by many http objects. This type of cookie is used by data that becomes irrelevant over time. While a permanent cookie might be used to track which language the user prefers.
Can you opt-out of being tracked?
For those who prefers to not be tracked, there are multiple ways to prevent it. The easiest way is to install a plugin in the web browser that blocks tracking scripts.