Linear regression is to fit a straight line to your data. In its simplest form, it is to find the slop (a) and the intercept b for fitting your data:

Here X and Y are one dimesional data arrays. If Y ~ a * X + b is a "good fit", then X can be used to predict or explain Y.

For details on how to build and evaluate linear regression modes, read the Electronic Statistics Textbook .

Excel has a standard macro, Analysis Tool Pak (Tools/Add-Ins...), for simple statistical analysis. The regression macro can be used for linear regression. The function linest (Insert/Functions...) can also be used for linear regression analysis.

For an application example, see How to calculate IC50.