In sports betting, a unit is a non-monetary measurement of the size of a bet. It is a way to measure success or failure in betting without showing the actual amount of money won or lost.
Units are used for accuracy and transparency, and they are more effective as a measurement system than the actual money won/lost because people are betting with different bankrolls, which can be thought of as a betting budget.

For example, someone who is betting €10.00 per bet and is up €100.00, has been far more successful than someone who is betting €200.00 per bet and is up €400. The first bettor is way ahead in units profited (+10), while the second bettor (+2) has won more money only because he is putting more at stake, not because he has been more successful.

What does it mean +XX units

If someone says he is +40 units, it means he is 40 times up on whatever 1 unit is to him.

Example:
If 1 unit is €1 for Bettor A, it means he is up €40 of profit.
If 1 unit is €10 for Bettor B, it means he is up €400 of profit.

Obviously, Bettor B has won more money, but that is only because he is putting more at stake with each bet. The two bettors are equally successful, as they are up the same number of units.

How Do I Pick My Unit Size?

Determine the Size of Your Bankroll
How much money should you deposit into your sportsbook? The answer depends on your personal financial situation and individual comfort level. Never bet any money that you can’t afford to lose.
Different people will have different definitions of what they can “afford to lose.” Be realistic in assessing your financial situation, as well as how much you can afford to hand over to an online sports betting site.

Choose a Unit Size
Once you’ve set aside a set bankroll amount, you need to determine what percentage of your bankroll you will wager on any one event.
Select a unit size somewhere between 1-5%. This allows you to stay measured and disciplined in your sports betting

Recommendation:
Keep the size of your bets consistent, using a standard bet amount for all of your plays.
Recalculate your bet amount at a fixed time frame. Every 1st of each month, or every 3, 6 or 12 months.
It’s completely up to you when you decide to schedule this.
Don’t start betting more (increase the unit size) when you’re on a bad run, in an effort to recoup your losses.