Explain the concept of baseline?

Base lines are logical ends in a software development cycle. For example let's have a software whose releases which will be done in phases i.e. Phase 1, Phase 2 etc. So you can base line your software product after every phase. In this way, difference between phase 1 and phase 2 can be tracked. Changes can be in various sections i.e the requirement document(some requirements changed), technical(due to changes, the architecture was needed to be changed), Source code(source code change), test plan change and so on.
Suppose there is an application which had undergone changes and was then baselined with each version. When the application was released, it was released with Ver 1.0 and baselined. After some time some new features were added and version 2.0 was generated. This was again a logical end so we again baselined the application. So now in case we want to track back and see what are the changes from Ver 2.0 to Ver 1.0. We can easily understand the same as we have logical baselines. After some time the application had gone through some defect removal i.e. Ver 3.0 was generated and again baselined and so on.
Base line is very important from testing perspective. Because testing on a software product which is constantly changing will not take you anywhere. So when the tester actually start testing, he/she need to first base line the application so that what he/she tests is for that base line. If the developer fixes something then create a new baseline and perform tetsing on the same. In this way any kind of conflicts will be avoided.