Plot appears to be the thorn in most novice writers writing lives. Many of my clients’ jaws drop to the floor when I break down what a plot and sub plot is.
Plot, in essence, is a sequence of interrelated events that influences, or somehow influences each other.
A sub plot is a secondary plot that is supplementary, or interrelated to the main plot.
Once you understand how to use plot/sub plot to raise the level of conflict in your writing, you will have fun with your characters.