When you press the play button, the simulation starts to run.
When you press the step button, the simulation runs for 1 time step or interval.
You can customize the height and width of the model by inserting numbers in the height and width box.
You may also choose the number of seals and fishes you desire by inserting whatever number you want in the seals and fish box.
Now, to actually change the height, width, number of seals, and number of fish you want, you will have to press the update button, after you have inserted the number of your choice.