The bottom toolbar mainly gives us an idea about the simulation. It displays the Simulation Time, which refers to the time that is passing within the simulator. The simulation can sped up or slowed down. This depends on the computation required for the current simulation.

The Real Time display refers to the actual time passing in real life when the simulator is running. The real time factor (RTF) is the ratio between simulation time and the speed of real time. If the RTF is one, it means that the simulation is happening at a rate identical to the speed of time in reality.

The state of the world in Gazebo can change with each iteration. Each iteration can make changes in Gazebo for a fixed amount of time. That fixed time is called the step size. By default, the step size is 1 millisecond. The step size and iteration are shown in the tool bar, as shown in the following screenshot:

We can pause the simulation and see each step using the Step button.

Before going to the next section, you can play with Gazebo and learn more about how it works.