A baseline is a collection of change requests that are all related to a particular milestone or release. Those change requests are in turn related to tasks and then to specific versions of files. Baselines therefore represent the state of all source files related to specific change requests at a given point in time. Baselines enable you to recreate the complete set of source files for specific milestones, from which you can then build and deploy.
In addition to change requests, baselines are associated with development packages, and in turn with release packages.