A unit is a percentage of your bankroll. Most money management schemes advise that you limit your plays to a small percentage of bankroll to prevent you from getting blown out at some point when you have a REALLY bad day (like I had today).
Your bankroll is how much money you are ready, willing, and ABLE to lose without if effecting your lifestyle or your mental health. For some people that's $100 total, for others its $10,000. Let's assume that your bankroll is $1,000 for the season. A unit should be somewhere between 1% and 3% of your bankroll, 1% is conservative and 3% is very agressive. So let's settle on 2% which makes your unit $20. So when someone puts up a 4 unit play and you're following their system you would bet $80.
If MOST PEOPLE are playing $100 units then I suppose they have a bankroll of $5000.
It is really NOT difficult to drop 15-20 units in a single day, it just happens sometimes, that's why it's so important to make sure that you can afford the loss.
Hope that helps.