This rating allows you to fly during official night, which is defined as the period between 30 minutes after sunset and 30 minutes before sunrise. This rating can be invaluable if you get delayed during a flight and need to land at night, rather than having to divert to a different airport. It is also a requirement to hold a night rating in order for the grant of a commercial pilots licence.
The night rating is a 5-hour course which includes a minimum of 1 hour of navigation and 5 solo take-off and landings as Pilot in Command (P1). The night rating does not expire, however in order to carry passengers at night you must have performed one takeoff/landing at night in the proceeding 90 days (unless you hold a valid IR which makes you exempt from this rule).