Planned Effort is the amount of hours expected to have taken place (up to the previous working day) based on the schedule.

Example:
I have a task planned over Monday - Friday (Total 35hrs / 7 hours per day)
By Wednesday, my planned effort looks like this:

Monday & Tuesday (2 days) have passed.
Based on the 7hr day x 2 = 14 hrs of effort was planned to be completed so far.

Note: If you have a fixed duration task with no Resources assigned this will show 0 planned hours.
Once a real or generic resource has been assigned the effort value will be calculated and updated.