In what context? When calling a SmartObject method? If so, then input parameters are essentially used as filters on Read an List methods to get only the records back that you want. Output mappings takes the result of the read/list and puts it into fields on your form or data fields in your workflow. If you meant something else, please clarify and I’ll help as best I can

Generally speaking, input mappings are parameters and output mappings are the return values.