Prompts are the most versatile Application objects in MicroStrategy since they provide flexibility to customize the SQL sent to the database, based on a user response to a question that is presented before Report execution.
The following is a quick reference table for this MicroStrategy Object:

Basically, when a Prompt is inserted in an object, MicroStrategy places a wildcard on the corresponding portion of the SQL. After this when the user responds to the Prompt, that unknown SQL gets replaced accordingly.