ELI5

A server is just a computer, right? It runs some specialized software to host a website. Anytime you go to visit a webpage, you’re making a request to the server that hosts that webpage. In response, the server will dig through its files, find the webpage you’re requesting, and send that page back to you. Some requests might be more complicated than that. All of the comments you see on a thread, for example, may be contained in a database somewhere. So the request could be; “hey, sort through ALL of the thousands of comments in your database, find the ones that are supposed to be on THIS thread, and send those back to my computer so my browser can show them.” Computers are fast, but they can only do so much at a time. Think about how your phone or computer works totally fine if you’re just playing a game; but how it may slow down if you have tons and tons of games and programs open. This is kind of the same thing. A server may work fine if it’s getting 100 requests a day. But then if you suddenly start sending 100,000 requests to it a day, that may be to much for it. It’s trying to do too much at once, and it will start to slow way down. That’s what people mean by “server load”. It’s basically “how much the server is trying to do all at once.”