Overlapping activity means that there are two activities overlapped in terms of date and time.
The system will not accept two activities for the same time, there can only be one activity for one specific period of time.
Eg: If you sync/submit an activity for 10am till 11am, then your second sync or submission states that the activity is from 10.30am to 11.30am, that is considered an overlap because you are supposed to be running from 10 - 11am and that is already one activity.
Note: If you sync/submit two activities for the same time period, the system will only take the first activity that is synced or submitted.