A plot outline can be described in a lot of fancy ways, but generally speaking it is a clear path for your story. That is, it is like a timeline of events that drive your plot in the direction you want. You can set up your plot outline in a lot of ways, but really all you need are the major (or minor) events laid out in the way you will write it in the final piece.
At least when you start out, having a plot outline can be extremely useful in the drafting phases. Depending on how long your story will be, it can quickly get confusing or overwhelming when it comes to knowing what comes next or remembering what happened earlier in the narrative. Writing out your events in a way that make sense to you can go a long way once you really start putting pen to paper.