It is simply that, estimated. When you don't know how long the task will take you "estimate" the duration. Otherwise, it is a known duration. Project does take it into consideration when calculating schedules. It can also be good tool to help manage unknown task timing.
Microsoft article about Estimated (task field):
Description The Estimated field indicates whether the task's duration is flagged
as an estimate. The Estimated field contains Yes if the task duration
is an estimate and No if it is a set duration. You can change whether
a duration is estimated or set by choosing Yes or No in the Estimated
Best Uses Add the Estimated field to a task sheet when you want to
view or change whether a task duration is estimated. Add this field
when you want to sort, filter, or group for whether tasks have
estimated durations. Sorting, filtering, or grouping based on the
Estimated field can be helpful when you want all the estimated
duration tasks to be displayed together so you can change the
estimates to set durations.