Basically, it is about 'bankroll management'.
The common theory among punters is that the best long-term strategy is to have a known bankroll, the amount you are willing to bet with over a period of time.
Whatever that bankroll is, a 'unit' is 1% of the bankroll.
So suppose you are willing to punt with $1,000 for the season, a '1U' bet is $10.
You might have a few good weeks and find yourself on $1,200 total bankroll. You can either stick to the $10 per unit or adjust it to suit your new bankroll.
I'm not sure what most people on this forum are doing in that regard.
Either way, by limiting yourself to just a few units of punting per match/round, you limit the wild variances which can happen with gambling.
It means you can win less on any given match/round, but also means you can lose less.
And anybody who has ever been on tilt after an unlucky loss, or gotten carried away after a big win and made silly big bets trying to 'ride it', knows how important bankroll management is.
For me personally, I had a very good weekend last weekend, ended up about 20 units or so. But for the season prior to that I was down almost 10 units (IIRC). The other benefit of this method is that it makes it very easy to 'keep track' of your overall success/failure.
tl;dr
For most punters on this forum 1U is probably $1-$10, and it is supposed to be 1% of their total bankroll, and the idea is to limit one's potential losses due to bad luck or bad betting. A confident bet will be around 2U, speculative bets will be more like .1-.5 units.