Tumen is like a big group of soldiers or warriors. Imagine you and your friends playing a game of tag and you have to team up to catch the other players. If you have five friends with you, that's like a tumen - a group of six people working together.
Back in the olden times, people in some parts of the world used tumen to measure the size of an army. So, if you had 10 tumen, it means you had 60 people working together - like having 10 groups of six players each.
The word 'tumen' comes from a language called Mongolian and was used by the Mongols in their army. They were really good at fighting and they used tumen to organize themselves and plan their battles. Nowadays, tumen is mostly just used as a historical term to talk about the Mongol army.