This is a new feature introduced within the opportunity line for the Project manager to build the estimates by adding Tasks, Labour, and Expenses. All those things can be done in one place.

Different fields which are part of Estimate Versions:

Fields Description
Version# When you create a new version estimate Version# will auto-populate.
Estimate Date Estimate date defaults to today’s date.
Description Description will be auto populated based on the description from Opportunity Line along with Version number.
Default If yes then this version is the default for me.
Include BI If you want all versions to be available, then Include BI should be yes.