What is structure?

Structure typically refers to how a story's plot is arranged. What happens first in a story's plot? This is usually called exposition. This includes how an author introduces characters and a sense of place and time (setting).
Next, the characters interact with one another. This section of a story is called rising action. The author introduces some major questions in the story that create a sense of conflict. These conflicts lead the reader to ask, "What will happen next? Will the main character(s) (protagonist) be successful? Will the opposing character(s) cause problems for the main character(s)?
This typically leads up to a climax, which generally happens at least midway into a story but can happen at any point in a story. This is usually the highest point of intensity in a plot line. It is where readers are faced with the most significant conflict(s) and problems of a story.
The final two parts of a story's structure include the falling action and resolution. The falling action begins to answer the reader's questions about the major conflicts. The main characters determine solutions to their conflicts, or discover that their problems may never be resolved. Either way, they find answers to the big questions of the story.
Finally, the resolution happens when the story comes to an finish. The major questions in a story are answered, but not always with a positive answer. Some problems may be resolved—others may never get to be resolved. The reader, however, is generally given answers to most questions in the story in the resolution.