A unit is the basic amount of money a bettor or casino player uses to place a bet. The idea is to describe stakes without naming a currency, which keeps gambling maths simple: nobody has to convert wins and losses back and forth between amounts.
Players usually set their unit once they know their total bankroll and how much they plan to spend. In most cases a unit is somewhere between 1% and 5% of the bankroll. From then on, they think in units rather than amounts, which makes a betting system much easier to follow. At a casino table where chips come in $1, $5, $25, $50 and $100 values, a player with a $5 unit doesn’t think “I’m betting five dollars” but “I’m betting one unit.”
Units in operator reporting
Tipsters and sportsbooks quote results in units for the same reason players use them: units normalise results across stakes. For a multi-currency casino, reporting in one base currency does the same job and makes markets comparable.