A linear regression is a line used to estimate the relationship between a dependent and an independent variable for a data set. In a diagram with a data set showing the height, of tomato plants after they have grown for weeks, the line estimates how fast the plants grow.
This line, called the line of fit, can be applied to a data set by hand with the help of a ruler. This typically results in only a rough estimate of the relationship. To get a more accurate estimate of the equation of the line of fit, other mathematical methods can be employed, such as the method of least squares.