It turns out that in Ferguson,"court fees and fines are the second largest source of funds for the city; $2.6 million was collected in 2013 alone."
The scenario often goes like this: a poor person is pulled over for a minor traffic offense like running a stop sign or speeding. They can't pay the fine because they haven't got the money. They get put in jail for not paying their tickets. When someone is put in jail, somehow the rest of the family manages to scrape up the money to pay the fines and get them out. So it turns out that jailing a poor person is a good way to get them to pay traffic tickets, and judges keep doing it.
In Ferguson, black people are disproportionately stopped for traffic offenses. What's ending up happening is that the city is raising a lot of money from giving minor traffic tickets to black people who cannot easily afford to pay them. This may not be what is intended, but it's the fact of the matter.
Read this New Yorker article Yorker article for more on modern-day debtors prisons.