The plot is the storyline, put simply, it is "what happens".
A plot can be simple -- a straightforward retelling of a set of ordered, related events. Or it can be complicated -- for instance, by telling the events out-of-order, by hiding key facts until later, or by interweaving several different sets of events.
Sometimes there is a plot or main story and one or more "sub-plots", that is related stories. An author may use the sub-plots to set up characters to be "in place" for later on in the main story plot. Sub-plots may also be used to contrast against the main plot. For example, a character making a moral decision in one plot may emphasize an immoral choice made by another character in a parallel plot.