It means that the report is from an automated weather station and information from sensors are not available for visibility etc.
From AIP Australia:
12.20.1 A report from a fully automated AWS that does not include information from sensors for visibility, weather or cloud will report ////, // or ////// respectively in lieu of these parameters.
For this METAR, it can be seen that there are a few clouds at 2100 (with further data missing) and there is towering cumulus, with further data (cloud amount and height missing). For example, decoding this METAR at flightutilities.com gives:
LONDON - STANSTED - ENGLAND
Report emitted the day: 03, time 15:20 UTC
Wednesday 3 February 2016 20:50 local time.
Report is fully automated, with no human intervention or oversight.
Wind: True direction = 290 degrees, speed: 11 knots (20 km/h) (6 m/s).
Minimum horizontal visibility: 10 km or more.
Clouds: A few (1-2 oktas), at 2100 feet above aerodrome level (640 meters), but it is unknown whether it is a convective cloud type.
Clouds: cloud type not reported, the cloud base is under the station level or not reported, towering cumulus.
Temperature: 7 degrees Celsius (45 Fahrenheit). Dewpoint: 2 degrees Celsius (36 Fahrenheit). Relative humidity 70.47 %. Wind Chill: 4 degrees Celsius (38 Fahrenheit).
QNH (Sea-level pressure): 1022 hPa (30.18 inches).
(I've removed some data and added emphasis).