Bundeskartellamt is like a big time referee in the game of business. Just like how referees make sure that all players play fairly and follow the rules, Bundeskartellamt makes sure that big companies in Germany play fairly and don't break the rules.
For example, if two big companies want to merge together and become one even bigger company, Bundeskartellamt will investigate to make sure that this won't hurt competition or make prices go up for people like you and me.
Overall, Bundeskartellamt helps keep competition fair and makes sure big companies don't bully other smaller companies or cheat their customers.