Transmission-based precautions are special steps that doctors and nurses take when they think someone might be sick in a special way that could spread to other people.
It's kind of like when you have a bad cold and you cough a lot, so Mom and Dad make you cover your mouth and nose so they don't catch your cold too. The doctors and nurses also want to make sure they don't catch or spread any sickness, so they take extra special steps to keep everyone safe.
There are three kinds of transmission-based precautions. One is called contact precautions, and that means the doctor or nurse has to wear gloves and special gowns so they don't touch someone's germs on accident. Another kind is called droplet precautions, and that means when someone coughs or sneezes, special masks have to be worn because the germs can travel through the air in little droplets.
The last kind is called airborne precautions, and that means when someone is super duper sick, they have to be in a special room with their own air system so germs don't spread through the air they breathe.
So, transmission-based precautions are like really special rules that doctors and nurses follow to make sure everyone stays safe when they are helping someone who might be sick in a way that could spread to other people.