A plot hole is when there's something missing in the story structure. If your character starts in a field, and the next scene is in a space ship with no explanation, that would be a massive plot hole. The reader would be lost. Now, on a smaller scale, a more common situation would be it's more about inconsistency. Something doesn't quite make sense in the sequence of things. This illogical situations can occur with the direction of the story or within a character who takes actions that don't make sense. Generally, people think of plot holes in terms of the flow of the story. Analogy: Picture a bridge over a river. You need to drive across it, but there's a segment missing in the center. That missing piece is the plot hole.