Hello Harini , "Wass up" is just a slangish short way to say " What is up" , which is not an expression but a question form to ask about what the other is doing or how things are with him/her or to ask about the current state of anyone , what he/she is doing ? what they like possibly to do ?? so in a slang way it would be "Hey man ,wass up ?" ;)

'wass up' is actually 'slang' (slang is like a shortened form) of what's up. Like asking what's new, or what can you tell me. do you understand?