Linear regression is a statistical process of constructing a model to explain the degree to which one variable's change (x) can be used to explain another variable's change (y). If the model fits the data well it can be used to predict values of y that can be expected based on values of x.

This video demonstrates simple graphical techniques for building a linear regression model in Excel and explains key outputs that excel generates.

For more comprehensive discussion of the statistics involved with building and interpreting regression models see this tutorial.