Rasoolollah means 'Messenger of Allah (God)'. This is one of the titles of a Messenger Prophet - A Prophet who is sent with a Message (usually a scripture) is known as a Rasool (Messenger). You will often find this word in one line of the Adhan - the call to prayer: Ash-hadu ana Muhammadun Rasoolollah - I bear witness that Muhammed is the Messenger of Allah.