Allah sends His Salat (Graces, Honours, Blessings, Mercy, etc.) on the Prophet (Muhammad SAW) and also His angels too (ask Allah to bless and forgive him). O you who believe! Send your Salat on (ask Allah to bless) him (Muhammad SAW), and (you should) greet (salute) him with the Islamic way of greeting (salutation i.e. AsSalamu 'Alaikum).