What are complications in a plot?
I assume that you are asking what the term "complications" means, and how to identify them, and not for the identification of complications in a specific plot.
Complications are, simply, additional elements that prevent the plot from going straight from A to B. They are also called conflicts. For example, if the plot of Macbeth were uncomplicated, then Macbeth would have killed Duncan without a second thought, and reigned as king: The End. However, Shakespeare introduced a variety of complications and conflicts to build tension and depth;
- Banquo sees the same prophecies that Macbeth does, but denounces them, and becomes an enemy to Macbeth
- Macbeth sees Banquo's ghost in public, challenging his sanity
- Macbeth's manhood is challenged by his wife
- Macbeth loses control of his power and becomes a despot
- The witches refuse to give straight, plain answers
And so forth. Each of these complications gives the characters a motivation or a choice to make; for example, when Lady Macbeth accuses Macbeth of cowardice, he now has the choice to accept her accusations and lose some of his self-confidence and his esteem in her eyes, or he can confront her challenge and discard some of his better judgment in the process. Had she not challenged him, or not even been present in the scene, the conflict would not have been so great, and Macbeth might have simply brooded over the subject instead of acting.