ETA (Estimated Time of Arrival) is the minimum number of days that the constraint is resolved before the start of the work without delaying it.
If ETA = 5, it means the constraint needs to be cleared at least 5 days before the start date of the task.
ETA can be edited at any point in time till the constraint gets done.
Changing the ETA will not affect the task start date at any point in time, but it can affect the constraint and condition due dates.
The minimum ETA that can be set is 0. It will mean that the task can be started on the same day when the constraint is done.
The following steps guide you to Edit ETA:
Double click on the Constraint (indicated by a circle and in-lined with its activity)
Click the Pencil (Edit) button
Edit the values in the ETA box
Click Tick
Check out the article 3.18 Activity Constraint ETA to know about ETA in activity constraints.