The first estate was the clergy, they were very very rich or wealthy, and they collected a tithe (tax) from the peasants which didn't have a lot of money.
The second estate was the nobles, they had almost all control over the peasants. They collected tolls from people on roads or streets. They didn't have to do military services and they didn't have to pay the most taxes.
The third estate was the peasants and arrcasints, they were forced to do military work and work for the landlords or the town. They didn't get paid for what they did, they did all the work for free and that's not fair because they also had to pay taxes, but they had no money because they didn't get paid from their job. They were the poorest group, and the richest groups didn't have to pay the taxes.