Let's say you log into an https website. All the data sent back and forth is secure from being snooped on or spoofed. One of the items that you got back from that website was a cookie so you don't need to provide your login credentials for every single page that you try to view. That cookie identifies you to the website so it will let you in and let you post as aldend123. Now say someone tricks you into going back to that website using an http url, perhaps by sending you an email message with links in it. Your cookie is now sent "in the clear" which is a security risk. Anyone who can see that cookie can become aldend123. So most modern browsers will refuse to send cookies that they got using https over a cleartext connection. It's clearly NBD for a website where people yak about motorcycles, guns, and Donald Trump, but it's a good idea to get in the habit of using https as much as possible because it's substantially more secure than http.
https://en.wikipedia.org/wiki/HTTP_cookie#Secure_cookie