But in josaa schedule given that report to reporting centre/admitted institute means?

Your present choice would automatically freezed and there is no need to report at RC .Admitted institute means which college you have been alloted.

Document Verification and Acceptance of Seat by reporting at reporting centre/admitting institutes- WHAT DOES IT MEAN?  

You have to report once at RC for document verification and if you desired to change your option from float to freeze or slide then go in mid of counselling otherwise your last round seat will automatically freeze.
For college admission you have to report at admitting institute.