IF_PRP_RESOURCES_AVAILABILITY is a standard SAP object class available within R/3 SAP systems depending on your version and release level. You can view/maintain the class details by entering its name into the relevant SAP transactions such as SE24, SE80 or even SE84. Below is the documentation available for class IF_PRP_RESOURCES_AVAILABILITY including details of:
A method is a coding block that performs a certain procedure (i.e. ABAP functionality) on an object within the overall SAP class. In simple terms if the object is an SAP database table a method could be the procedure to add a record or to delete a record. If you are new to OO in many respects, the implementation of a method is similar to a function module and can be called in a very similar way using CALL METHOD.
Instead of "CALL FUNCTION", Methods are referenced using the following syntax: CALL METHOD =>methodname EXPORTING/IMPORTING....