What does the error ‘502 Bad Gateway’ mean?

Hello, I’ve been trying to use Sci-Hub as usual for my assignment but it’s not working. It keeps presenting me with a blank page that says “502 Bad Gateway”, I’m not very tech savvy, can someone please explain what it means?

I tried to use the other website mirrors in the wiki section of this thread but they all showed me the same error. [sci-hub.se](https://sci-hub.se) loads the home page of the website as normal but when I paste the DOI of the paper, it shows me the error then.