What does the Planned cost column in the Hierarchy tab indicates? Is it the Forecast which we see in the Cost plan ?
The Planned Cost values in hierarchy should be same as Financial plan Cost Plan or Planned Cost from Financial Summary Page.
Yes, it is the total planned cost from the Planned of Record cost plan of your investment. Or the total planned cost in your simple budget if you don't have a cost plan.
Can it be defined in this way " It is the Total Forecast entered from Project start date to End date " ?
I would say, "the total forecast COST of your Plan of Record Cost Plan" (note that the start & end date of the cost plan can be different than the project start & end date, if it's not the case for you, then you can say Project Start & End).
Forecast can hold other values than cost (e.g.revenue).