A timeline in Excel is used to represent a chronological sequence of events. It can visually display important dates and milestones on a horizontal or vertical axis, allowing users to quickly understand the sequence of events and their durations. Timelines can be applied to many industries and projects, such as project management, historical research, and marketing campaigns. Excel provides various tools and templates to create different types of timelines, including Gantt charts, milestone charts, and chronology charts.