It means one server received an invalid response from another server it was accessing, while attempting to load the web page. You can't do much to stop it happening. The servers need to be sorted.
As had been indicated. '502' errors aren't usually 'at your end'. If the problem is with a server that your computer is trying to connect to (rather than with your own computer or with your internet connection) you can't fix it.
If you can get access with Firefox, but not with Chrome,them it's definitely a problem with Chrome. (See the suggestions in my link above for possible solutions).
If you still experience problems with Firefox then either your security software is blocking access (check the settings) or there's a problem with Windows (try running System Restore).