Suppose you have imported table from database as a source and you have used that source in multiple mappings. If there is any change in the source table structure in the database then how will it be reflected in the mapping?

When you import a source table in Informatica, it creates a static definition of that table structure at the time of import. Changes made later in the actual database are not automatically reflected in the imported source definition or mappings that use it. You must manually re-import or synchronize the source definition to pick up database changes.