Android's media scanner process is a service which maintains a list of available media files on an Android device. This could include music, video, or pictures on the device's physical storage.
Files must be indexed by the media scanner in order to be accessible to media apps (like music players). If you have added files to your device but they are not available in your apps, then it could be due to an issue with the media scanner service, or the service may need a chance to run and notice the new files.
Not to be confused with document-scanner, which is related to scanning paper documents into digital formats.