This wording means that the vehicle must be garaged between the hours of 10pm and 6am whilst the vehicle is at your home address (or other address agreed with us) for theft and malicious damage cover to apply. This only applies to your home address so if you are away from home (at a hotel/friends etc) you are covered as normal. (This answer is for the schemes and may be slightly different with some of our "normal" motor insurers).
Dependant on your circumstances this cover is not always included. If you do have this cover this is noted on your certificate of motor insurance. Please note that in most circumstances this cover is restricted to Third Party Only cover, so you will not be covered for the car you are driving, only damage you would cause to third parties.