Read this resource, which focuses on optimizing using project resources and time required to complete a project.
At this stage, the project schedule should be optimized to achieve the appropriate balance between resource usage and project duration to meet the project objectives. Once the project schedule is established and agreed upon, the project schedule becomes what is known as the baseline schedule.
We will measure our progress during the execution of our project against this baseline schedule. We can do this in several different ways: comparing calendars using tracking gantt charts, earned value metrics, etc.
This baseline is what the Project Manager uses to judge performance and will look carefully for deviations from this baseline as the work progresses.

Source: J Scott Christianson, http://web.archive.org/web/20160129201253/http://www.projectmanagement.ninja/topic/schedule-baseline/ This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 License.