it means that whom ever says this wants to have s*x with your mum. 'ur mum' is an expression used to agree or nuetralise a conversation.
' haha you thought that was funny, that was sh*t' then in responce ' ur mum'
'word' is an agreeing word, if you agree with something you say 'word'
"omg your mum is so fit"
"word"