Alright kiddo, let's talk about tannaim. Tannaim were really smart people who lived a looooooong time ago in Israel and they were experts in Jewish law. They would have discussions and debates with each other about how to follow the rules in the Jewish religion.
It's like when you have a game and you and your friends want to make rules for how to play it. The tannaim would do the same thing for the Jewish religion. They studied the Torah and other Jewish texts to figure out what the rules were and how to follow them.
Some famous tannaim we know about today are Rabbi Akiva and Rabbi Hillel. They taught lots of people about the Jewish religion and made a big impact on how people follow it today.
So basically, tannaim were really smart people who studied Jewish law and helped create the rules we follow in the Jewish religion today.