So, imagine there is a group of kids who love to play soccer. Some of them are really good and want to represent their country by playing on a national team.
But, a long time ago, the country of Germany was split into two parts - East Germany and West Germany. This meant that there were two different soccer teams for each part of the country.
The East Germany national under-21 football team is a group of young players from the former East Germany who are really good at soccer and want to play for their country. However, they only play against other teams in their age group (under-21 means they are all younger than 21 years old).
Even though Germany is now united and there is just one national soccer team, the East Germany under-21 team still exists as a way to showcase the talent of young players from that part of the country.