A “Finish-to-Start” mandatory dependency means that a predecessor task must be set to completed before its successor may be started. If a lag time is also provided on the “Finish-to-Start” mandatory dependency, that time will need to elapse (after the predecessor has been set to completed) before the successor task may be started.
In AceProject, “starting” a task means changing its status from a “Waiting” status to an “In Progress” or “Completed” status.
You may find further information on how AceProject dependencies work in this help page.