The "mousemove" event is a type of event in web development that occurs when the user moves their mouse pointer over an element on a webpage. This event can be used to track the position of the mouse and respond to user actions, such as displaying tooltips or changing the appearance of elements. When a "mousemove" event is triggered, it provides information about the mouse's current position, including its coordinates on the screen. Developers can use this data to create interactive features, enhancing user experience by making web applications more dynamic and responsive to user input.