Swimlanes let you describe where the responsibility of a step in your process lies.
Swimlanes are shown as horizontal bands on the simulation window. In the example below there are 3 swimlanes, one of which is highlighted by a red box.
Simul8 automatically creates swimlanes for you whenever you drop a BPMN object onscreen outside an existing swimlane.
To change the height of your swimlane hover the mouse over the bottom line in the swimlane, the cursor will change, grab the line and move it up or down to resize the swimlane.
Swimlanes are largely a visual tool but they do have some properties.
You can access these from the Groups button in the Data and Rules tab. Here you can specify: