A unit is your standard bet size based on your bankroll. For example, if you typically bet $50 per game, one unit (1u) would equate to $50. A good rule of thumb is to use 3-5% of your starting bankroll as your unit size. For example, a $50 bettor should have a bankroll of approximately $1,000 to work with.
When tracking bets or analyzing your betting history, the "u" next to a number stands for unit. In the example shown below, this user has a record of 15-16-0 on NFL bets with 33.37u. The highlighted green "33.37u" indicates that this user has won 33.37 units over a total of 31 NFL bets.