HUD in online poker stands for Heads-Up Display and is a useful tracking tool players use to get an insight into their opponents' playing tendencies. The HUD displays helpful numbers and percentages next to every player, directly showing you how your opponent has acted in similar situations before.
HUD Poker Example:
A good poker HUD contains valuable information like VPIP (Voluntarily Put Money Into Pot), PFR (Preflop Raise), WTSD (Went to Showdown), WSD (Won Money at Showdown), 3-Bet Frequency, and AF (Aggression Factor), among many others.