What is set parameter and get parameter?

We can pass data to a called program using SPA/GPA parameters. SPA/GPA
parameters are field values saved globally in memory. Each parameter is identified by a
three-character code: you can define these parameters in the object browser by
selecting Other objects on the first screen. The SPA/GPA storage is user-specific and
valid throughout all the user's sessions.by using the SET PARAMETER or GET
PARAMETER statements
These statements let you store and retrieve SPA/GPA values from an ABAP/4 program.
If the selection screens for the two transactions do not share the same required fields,
use these statements to store screen fields explicitly by name.
Before calling the new transaction from a PAI module, store the caller transaction's fields
under one name:

SET PARAMETER ID 'RID' FIELD .

The system stores the value in in the SPA parameter 'RID'. The three-
character identifier 'RID' must be defined in the SAP table TPARA. If the SPA parameter
'RID' already contains a value, the SET PARAMETER statement overwrites it (with the
contents of ).
In the PBO module for the called transaction, retrieve the fields under the other name:

GET PARAMTER ID 'RID' FIELD .