1. What cookies are
A cookie is a small text file that a website asks your browser to store on your device. Each cookie contains information such as the domain it belongs to, a lifespan, and a value. When you return to the same site, your browser sends the cookie back so the site can recognise your device and remember things such as whether you are signed in or what preferences you have set. Similar technologies include web beacons, pixels, local storage and tags, which work in comparable ways.
Cookies are not programs and cannot run code on your device or read other files. They are simply data that a website stores and later reads back. They are used for convenience and measurement — for example, keeping you logged in, remembering a theme you chose, or counting how many people visited a page. They are not a complete security system, and the security of your account depends on a combination of encryption, access controls and good practices, not on cookies alone.