Single-particle trajectory is a way of studying the motion of a single particle (for example, an atom or a molecule). It tells us where the particle goes, how fast it is moving, and how long it stays in one place. To study this, scientists use a tool called a 'particle tracker'. They use this tool to follow the particles movements over time. Think of it like having a camera that can watch a particle and record what it does. Scientists can then use this information to learn more about the particle and the environment it is moving in.