Tutorial 3 presents you rigid bodies and tracers.
A rigid body is an idealization of a solid body of finite size in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it.
A tracer is a tool which shows the trajectory of a given point on a rigid body.
When a rigid body (here a disk) is selected you see three grey handlers on it. Using them by clicking on them and moving them, you can change the velocity, the angle and the angular velocity of the body.
The experiment in Tutorial 3 shows a disk and a box linked by a spring. A tracer (the blue one) is already on the box. You can add a second one: select Tracer in the Palette panel then click on the box on the point where you want the tracer to be. In the Properties panel, click on the color line and on the right of this line you can click on the blue square and a color palette appears: you can choose a new color for the tracer. The screenshot above shows two tracers after the simulation is run for a few seconds.