Add a short explainer on what the world simulation is

Just so this section doesn't launch straight into instructions.

Author: PeterJCLaw

simulator/setting_up_simulator.md

@@ -87,6 +87,8 @@ If you're still having problems, ask for help in [Discord][discord].
 
 ## The world simulation {#world-simulation}
 
+The world simulation we provide defines the environment that contains the simulated arena and robot as well as the SR API for use in the simulator.
+
 1. Create a directory, perhaps called `simulation` where you will store your robot code.
 2. [Download the simulation](https://github.com/srobo/competition-simulator/releases/download/sr2024.1/competition-simulator-sr2024.1.zip), the latest version is sr2024.1, released on 2023-10-21.
 3. Unzip the simulation as a folder inside the folder you created in the first step: