HTTP:

HTTPS:

As you can see from the difference between the two screenshots, images like the medals on a user profile don't load with https. They return a 403 forbidden if accessed through https. Clearly some server-side misconfiguration, but the forum works just fine if accessed through http. If you have https-everywhere as an extension (or browser setting?), turn it off or make an exception for this website.
The difference between https and http is that with https (the s stands for secure) the traffic between the website and you is encrypted, meaning nobody in your public wifi or evil three later agencies tapping the wires at internet exchanges can read what you're sending or receiving. Most notably this means any passwords you send can't be intercepted. Nowadays it's considered good practice for all websites to have https.