A trend line is also known as the "line of best fit". It basically is the line that covers many of the data points that are in a scatter plot and stands as the average change over time. The data points are simply one plotted point of information, and only when you have all of them do you gather information, the trend line is a complete summary of a single scatter plots information all on its own. Hope I helped:)
Describe a trend line in a scatterplot and how it should look in comparison to the data points.