The “switchable” type is defined in CoreTypes.aslx.
It marks the object as switched off (switchedon = false), and adds “turn on/switch on” and “turn off/switch off” verbs to the object. These provide a default implementation so that sensible responses are printed when the player tries to switch on an object twice etc. The verbs simply toggle the state of the switchedon attribute. You can use onswitchon and onswitchoff to trigger events in the game when the player switches the object on or off.