Glad you found it. Pre run basically goes back in time and acts as if your particles were already born prior to the layer starting. For example, if you want some snow in a scene and want it in the process of falling at the start of your comp, you would have to make your particular layer longer than your comp and then push it back in time so that your snow fell into the scene prior to the start of the comp. With pre run, you dont have to do that. Just increase the slider and Particular calculates back in time and acts as if particles were emitted before frame 0 of your comp.
Hope that is relatively clear and answers your question.