If you planned to start your project on January 1 and created a big plan, but after that decided to go hiking to New Zealand until February, you would want to move the whole project somehow. You can use some tricks to do it in the old good GanttProject 2.0.10, but starting from build 428 of GP Praha you have an explicit option.
In the Calendar page of Project Properties dialog you see a date chooser. It shows the current project start date, that is, the start date of the earliest task in your project. Choosing another date will move some or all of the tasks, depending on what rescheduling option you select.
Move all tasks will just add the same number of days to all tasks in the project, and it is the default option. However, sometimes you want to move the project start but prefer to keep the remaining tasks intact whenever possible. You can use the second option for that.