UTG = Under the Gun, the first person after the blinds. First person to act pre-flop and third to act after the flop if the blinds play.
UTG+1 is the one after
UTG+2 the one after UTG+1
MP1 = Middle Postion, just after UTG +2
MP2 = like above
MP3 = like above
CO = CuttOff seat, not sure why it's called the Cuttoff but its just before the Button
The Button = The dealer button, the very last player to act on almost all rounds of betting apart from Pre-flop which is closed by the blinds unless they raise.