Biosecurity in the United States is like a big wall that protects our country from tiny things we can't even see, called germs. Just like how we wash our hands and cover our coughs to keep from getting sick, the United States has rules and ways to keep bad germs out of the country.
Have you ever seen a checkpoint where cars have to stop and show their ID before going into a special place, like a museum or airport? That's kind of what it's like at our borders. When things come in from other countries, like people, animals, or even plants, they get checked and sometimes even have to go through special machines to make sure they're safe. If something bad is found, they can't come in.
There are also people who work really hard to make sure that the germs in our own country don't spread too much. If there's a big sickness going around, these people might tell everyone to wash their hands more, or even close certain places until the sickness goes away. This is all to keep us safe and healthy.
So basically, biosecurity is like a big protective force field around the United States. It helps keep us safe from bad germs and things that could make us sick.