These are once again reporting specific functions. For example, the notation {Orders.Order Date} is used to refers to the value of the field in the current record whereas Previous ({Orders.Order Date}) refers to the value in the immediately preceding record and Next ({Orders.Order Date}) in the next record. IsNull ({Orders.Order Date}) checks if the field's value is null.
Other examples are PageNumber and TotalPageCount that you can use to access pagination information about your report.