This might initially seem like the sort of question that is easy to Google, but it's not. Instead of useful results, you get things like answers to the question "What does Ubuntu mean?", or "what does ~ mean in a Linux shell?" or "mean, median, mode, and range". Every different attempt I've made fails because of the normal meaning of "mean" and how Google interprets that.
The context in which I've encountered this is my Digital Ocean control panel: