Kapp stands for Kinetic Application. At its most basic, a kapp is meant to be a specific application within a Space. Examples of kapps are Queue (a tasking appliaction) and Services (a Service Portal application).
Since the main component of a kapp is its forms**, the kapp has to provide the infrastructure to support them. In general, this includes:
Another key part of a kapp is how it's displayed. This is normally set in the Settings portion of the kapp, and includes locations for themes, attribute definitions, and other kapp specific support settings.
** a kapp like Calendar or Tech Bar, may have a slightly different focus, but the principles are the same.