Biocontainment is like a very special room where we keep very dangerous germs and viruses trapped so that they don't get out and make people sick. Just like how we keep toys in the toy box and cookies in the cookie jar, we keep the germs in this special room so they don't get out and cause trouble.
The room is designed so that air and water can't escape outside, and people can't accidentally bring the germs out when they leave the room. There are also special suits that people wear to protect themselves when they go inside the room to work with the germs.
It's like if you were playing with some really messy mud, you might wear a special suit to keep the mud from getting on your clothes and making a big mess. In biocontainment, the suit keeps the germs off of the people inside the room.
Keeping dangerous germs and viruses contained in this special room is important because these germs can make people very sick or even hurt people. By keeping them in this special room, we keep everyone outside the room safe and healthy.