What is a Baseline

When all of the Requirements have been reviewed by the stakeholders and approve a baseline set of requirements is established.
The baseline is the starting point for your planning of both time and cost. This is the starting point but as we know it will change. As we move forward with the project we will find things that we missed and needs may change. This is ok as long as you started with a good baseline. The baseline allows you to analyze the impact to your program for both cost and schedule to determine if a change is cost effective. Without the baseline as a starting point there is no way to determine what the impact will be.
The baseline forms the basis for all work performed on the project without limiting the possibility to explore arising opportunities.
Maintaining the approved baseline through all changes and amendments is generally the requirements manager responsibility. All to often I find that programs do not have a dedicated requirements manager and that It is left up to someone as a part time job.
Changes will come from stakeholder inputs and user priority changes. Users may think they know what they want but will spot deficiencies in Business Requirements and propose improvements based on new understanding of what they can do.
Maintaining a baseline requires that any proposed changes go through a formal change request process to insure that all stakeholders have reviewed them for impact on requirements baseline. This impact analysis review should identify any impact to , scope, schedule, cost, contract, and work in progress. The impact analysis may identify that further evaluation is required and a repetition of the Requirements Management Process.
Baselines are not static but are a changing thing, but they are a starting point to measure form. All races have a starting line and baselines are that for development projects.