If a simulation is running and you don't want to wait for it to end, it can be stopped by selecting this menu item.
Usually you use this because you expected the simulation to have stopped already, but it hasn't because of some error in the circuit and because the simulation time limit is set too high.
The default time limit is 100,000,000 time units. To change it, select the show simulator menu item and change the value in the time limit box at the top right of the simulation window. You can then hide the simulation window or simply run the simulation again using the Run menu item or the Restart button at the far left of the simulation window.