Cookies can most easily be explained as leaving a small trail in your browser confirming where you have been on the internet or navigation from certain emails that you open.
A cookie is a small file typically consisting of a string of letters and numbers that is sent by a web server to a web browser (e.g. Internet Explorer), and stored by the browser. This small file is then sent back to the server each time the browser requests a page from the server.
Cookies can be used by web servers to distinguish and track users as they navigate different pages on a website and to improve the experience for returning users.
Cookies can be either session cookies or persistent cookies. A session cookie will expire at the end of the user session, effectively when the web browser is closed. A persistent cookie will be stored by the browser and will remain valid until a set expiry date (unless deleted by the user before the expiry date).