You might have heard of the terms early start, early finish, late start and late finish. I will explain here in a very easy way to help you understand what early and late dates are.
Say project start date is 1 March 2019.
I have one activity in my project. This activity can start on 1st March. So, the earliest date this activity can start is 1 March 2019. This is its early start date. Now, say duration of this activity is 10 days. So if this activity starts on 1 March 2019, the earliest it can finish is 10 March 2019. This is its early finish date.
Say my project's end date (deadline) is 30 March 2019.
I have one activity in my project. Duration of this activity is 10 days. Now, the latest date this activity can finish is 30 March. This is its late finish date. If this activity was to finish on 30 March, having 10 days of duration, so the latest date this activity will start is 20 March. This is its late start date.
I hope you understood now the difference between early and late dates in schedule.
Late dates are usually not used. Someone might just look at the late date information, in case there is an activity he wants to start as late as possible in the project and also finish as late as possible.
If you are using Primavera P6, you can add columns early start, early finish, late start, late finish to see this information.