Alright kiddo, have you ever played a game with your friends where you trade toys or candy, and sometimes one friend gets more toys or candy than the others? Well, imagine that game, but with money and companies.
Back in the olden days, there were some people called "industrialists". They were very rich people who had a lot of money because they owned big companies that made things. These companies made a lot of money because they were able to make things really cheaply and sell them for a lot of money.
Now, some of these industrialists were called "robber barons". They were called this because some people thought that they were stealing from regular people. These robber barons would use all sorts of tricks to make more money and keep other people from competing with them.
For example, they might buy up all the companies that made the same thing as them, so that there was no competition. Or they might make secret deals with the people who ran the railroads (which were really important back then) so that they could ship their stuff more easily than other companies.
The robber barons also often didn't treat their workers very well. They made them work really long hours for very little pay and sometimes even had them working in really dangerous conditions.
So, to sum it up, a robber baron was a very rich industrialist who sometimes did bad things to make more money and who didn't always treat their workers very well.