Baseline
Gantt baseline is a snapshot of the current positions of tasks and milestones in the Gantt timeline, which can be placed alongside the chart at a future date. You can use the baseline to compare the current state of Gantt with the state at the time the baseline created
Create a baseline
Click on the Baseline and select New baseline then enter the name and create
Enable baseline
Click Baseline and find the baseline you want to enable and select it
Deselect the baseline to disable it
When a baseline is enabled you can confirm the baseline bars on the Gantt timeline
In the screenshot the gray thinner bars are the baselines, you can customize the color in board settings.