“U WOT M8” is a shorthand for “You what, mate?” which is a colloquialism used by English speakers in the United Kingdom, Australia and New Zealand.
It is typically used in response to an incoherent proposition put forth by someone else, along the similar lines of “what did you just say?” or wat or what is this. Alternatively,
the phrase can be used devoid of any context to confuse anyone that is unfamiliar with the expression.