A baseline at its core is a snapshot of your project at one point in time. You can use the baseline to compare your current project to that baseline, to see how the project is progressing compared to the original plan.
Two separate functions are present in the baseline feature of P6; Maintaining Baselines and Assigning Baselines. Simply, baselines are created in one place and assigned to the project for comparison in another place. This is separated because there are typically two different roles or users who would perform these functions as part of a balanced quality check. Since a baseline is used as a calculation for variances, the capability to create your own baseline is risky. For example, if original duration is six days and you realize there have been eight days on that activity; the variance between your original duration in the baseline and the current project actual duration is -two and you are late. If someone were to change the baseline to have eight days...