Stands for Common Weapon Sight. It is a Generation 3 (some may still be Gen 2 sights awaiting upgrade, but there shouldn't be many) night sight with an injected graticle. It has a 200m effective range at night.
It replaced the IWS, which was a lot bigger and more cumbersome but was a better sight (it a had higher gain).
The big brother of the CWS is the Maxikite. It is a brand new purchase and is a monstrous great thing used in sniper teams. Has an effective range of 600m at night.