It means you do not have to remain at 250kts or below whilst on the SID, and can accelerate now, or whenever you want. You do not have to, but can if you like.
As far as I am aware it does not mean you can ignore noise criteria (point of and radius of turns etc) although from heathrow as I recall most of these are fairly immediate on departure (Staines bit on DVR off 27?) and so probably finished by the time we lift the speed. You do not have to wait until 6000ft.
Even though I believe the noise tracking remains the pilot's responsibility, Gatwick departures to the west off easterlies and to the east of westerlies, we are not supposed to remove the speed until the initial turn has been made to 'assist ' in keeping the plane within the noise profile, although these turns are far tighter than any heathrow turns (except possibly CPT off 09).
Also the thread on 250kts presently running may negate all this as we are to trial 'not routinely lifting the speed restriction on departure' in the TMA...