3rdKey for safety
They should show you public information, but hopefully, the will show your data to only you.
Browsers should prevent servers from accessing any data on your device.
When you log into a site, you give it your password, and the site verifies it is you. But in order to not have to ask you for your password on every page, it needs to save somewhere "proof" that you're logged in securely.
A cookie file is the best way to do this (More about why we need cookies)