Data is encrypted when you access a HTTPS site, but not with HTTP. This means a third party can view your connection to the site, and any information submitted by you.
The connection to HTTP can be exploited, and malicious script can be injected to the response you (the user) receive from the server. This can include ads that drain computer resources (memory, CPU usage), or attempts to exploit vulnerabilities within your operating system/browser. As there is no authentication used with HTTP, there is no way for the user to know if the request/response has been changed.
It is considered generally safe to just browse these sites without inputting sensitive data, but the above is something to be aware of.
Reminds me of when I was looking up an issue for Kali Linux and saw like 10 different reddit threads, all full of “don’t use Kali Linux unless you know how to use it”. Which was very helpful when I had to use it for my coursework.
Side note: the most downvoted comment contained actual advice that fixed my problem.