In a basic sense, plot is the action in your story. It's what happens to the characters in the settings, the events that take place between the beginning of the story and its end. Scratch beneath the surface, however, and you'll find that plot is a lot more than just a series of events. A successful plot, like all other elements of a story, must meet certain requirements.
Before we consider what those requirements are, let's look at why events alone don't constitute a plot. Events are merely things that happen. Waking up in the morning, pouring a cup of coffee, and checking your email are some examples of events. Would you read a story about a person who did that and only that? I know I wouldn't. By themselves, these events are boring and insignificant.
So, what's the difference between a series of events and a plot? A plot is made up of a series of events, but those events are significant. They help describe the characters, convey the settings, further the action and illuminate the themes of a story. Events in a plot never 'just happen.' Every one of them fulfills at least one of the purposes I listed above, and the strongest events in a plot manage to do all of those tasks at the same time. It may seem like a very tall order at first, but, like everything else in fiction writing, it gets easier the more you write.
Sometimes, you'll find inspiration for your plot from the characters and settings you've already created for your story. If, for example, your main character is terrified of spiders, including a scene where she must deal with spiders in order to save the life of a friend (or her own life) would show your readers that she is able to overcome her fears. If the room full of spiders happens to be in the run-down house she inherited from a deceased relative, the readers get a strong sense of the setting. If the main theme of your story involves Good triumphing over Evil, then that plot point could be some foreshadowing of the major confrontation between the heroine and the villain.
It's important to recognize which events are important to your plot from those that are not. Your story should only contain the events that are necessary. When you edit your story, ask yourself why each event happens. If you can't answer that question by mentioning at least one of the requirements I listed above, and preferably all of them, remove that event from your story. Streamlining your story by taking out all the superfluous happenings will make it a much more satisfying reading experience. Don't be afraid to cut out things that serve no purpose in your work. Save any of the scenes you remove for later, just in case those events are better suited to another piece you might work on.
Plot is one of the most important threads in the tapestry of fiction. To practice crafting plots, write a one-sentence summary of a story, and then list the events you feel must happen in order for that story to have a beginning, a middle, and an end. Repeat this process as often as you want.