WAT means "WHAT" and it is used when someone does not understand what is being said or when someone is very confused
wat means "What About Today" and it is used to ask if someone can go out with you
dude: Hey sir, WAT?
sir: Idk I'm busy with home
dude: If you say so
Wat basically means and sounds exactly as what does. Because wat IS what.
R1: 'Wat up man!
R2: 'Wat?'
R3: 'He said Wat up man!
A word used by intellectual's meaning what
John said what
Me an intellectual: WAT