If you visit this site you will know what browser cookies are and that they are used to store information about you across visits to that website. A typical website will store a few to a few hundred cookies on your computer.
Each cookie has an expiry date for when they are no longer valid and then automatically deleted by the browser. Did you know the lifetime of some of these cookies are several decades in the future and that they are routinely used by data brokers and analytics companies to gather data on you?
That's why I've recommended installing a plugin like PrivacyBadger, to prevent these tracking cookies from being passed back to websites that track you across websites.
But you can also override the maximum lifespan of these cookies. I've experimented and 10-14 days seems to be a good medium between having to login to all your websites all the time and maintaining privacy.
On Firefox, open about:config and search for network.cookie.lifetime, you will find network.cookie.lifetime.days (set this to 14) and network.cookie.lifetimePolicy (set this to 3 to override the default behaviour or letting the website decide).
On Chrome or Vivaldi browsers it's possible to set these values by installing my FreshCookies addon, which does not contain any tracking itself.