Exactly what it says.
If you scroll down to the bottom of various pages on websites you might see a phrase like "22 queries, took 0.023ms to fetch". If you code badly (or add too much in) and have a cheap server both numbers will go up, if you also end up with a popular website (be it in general, because something interesting happened on your site or someone big linked it for some reason) then the number of queries shoots through the roof and people start waiting longer and longer, eventually timing out (or maybe hitting refresh if it takes a few seconds and the process repeats again).
To that end wait a while. Hopefully whatever is causing the extra load dies off and then you can visit. If it keeps up then the owners either get to improve their code, strip out elements of it to reduce load (why things might stop animations, stop a shoutbox, stop internal searches, reduce information displayed compared to normal and so on), buy a better server, set up a CDN to take some load, in some cases maybe buy some extra server ability (modern stuff makes this a bit easier in some ways for some setups) or just become known as a slow website. If you just need some data then chances are you came from a search engine and most of those will have a cache of stuff on it. You might also find something on
http://archive.vn/ https://web.archive.org/ or copied somewhere else.