Question: What is a plot structure?
Think of a plot as a series of events (things that happen).
If they were just random events, they would have no meaning.
A series of events linked by cause and effect, on the other hand, has a meaning. You can see that the initial event (perhaps a choice or an action) eventually leads to a certain outcome. Consequently, that first event has a significance.
However, a good writer will build a certain structure into the chain of events in order to create an emotional effect on the reader. The general pattern of a dramatic arc is...
setup (potential for trouble) --> complications --> crisis --> resolution
Now you have a plot structure that draws in the reader, builds tension, and then relieves it. The story as a whole shows a meaningful change that resolves a problem or imbalance.
In addition, the transitions between these four phases may be marked by significant turning points. Well developed story may also have a number of different dramatic arcs that unfold in this way (subplots, throughlines, character arcs, etc.). Each scene may have its own arc.
Each event in the plot plays an important role in the overall structure, so that if it were missing the story would feel less satisfying. Similarly, events that do not contribute to the structure will lessen the emotional impact.
Hope that clarifies things.