The best-fit line attempts to fit a straight line through all the points on a scatterplot, minimizing the sum of the squared distance between the line and the points. It is equivalent to a simple regression of the selected column (Y value) on the covariate (X value).
To view a best-fit line on a scatterplot:
The last option will display the best-fit line only if the correlation coefficient between the two columns is statistically significant.
See also: