Activity diagrams are visual representations used in software engineering to illustrate the flow of activities or tasks within a system. They help to map out processes, showing how different actions are connected and the sequence in which they occur. By using symbols like ovals for start and end points, and arrows to indicate the flow, these diagrams make complex processes easier to understand. These diagrams are particularly useful for business analysts and developers when designing systems or workflows. They can highlight decision points, parallel activities, and the overall structure of a process, ensuring that everyone involved has a clear understanding of how tasks are interrelated.