The phrase "to pop up" means to appear suddenly or unexpectedly. It can be used in various contexts, such as when referring to an event, a person, or even something that happens on a computer screen. For example, if someone walks into a room, it could be said that they "popped up" there. Similarly, if a notification appears on your phone screen without any warning, it is described as having "popped up." The phrase can also refer to an event or activity that happens spontaneously, such as a pop-up shop or a pop-up restaurant. Overall, the phrase means to appear suddenly and unexpectedly in various contexts.