system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does "&amp" sometimes show up on websites?
gpt
HTML uses characters like ampersand, greater than, less than, etc. in its syntax. So if someone wants to use those symbols in their web page, when they're putting the text in the HTML they need to put "&amp" for an ampersand, "&gt" for greater than, etc. When the browser displays the webpage, it understands to replace those with the proper characters.

The problem is that most webpages now aren't only HTML. There's PHP, Javascript, etc. and all these might not process the text in the same way. So a developer might put "&amp" in thinking that they need to do that, but they've made a mistake and that bit of text is actually not parsed in the same way HTML is so it never gets replaced.