Parameter ID is a three character ID that represents specific SAP fields. For example, BUK is associated with Company code, GJR is associated with Fiscal Year and so..on. This saves a lot of time during transaction entries and would avoid potential data entry errors.
How to find out the Parameter IDs in SAP? on Any screen field, Press F1 and click on Technical Information.
You can either go to transaction SU3 or follow the path System ->user profile -> own data -> parameters and then give the parameter name and the parameter value to maintain the values.