Alcohol is a pretty unique substance because it is not changed when we absorb it into our system. It stays intact through our digestive system and then gets absorbed into our blood. When our blood goes through our lungs to pick up oxygen and drop off carbon dioxide, it also drops off some alcohol, which evaporates out of the blood into the air we breathe out.
This evaporated alcohol is what is measured in the breathalyzer test. There is a direct proportion to the amount of alcohol found in our breath to how much is in our blood, so we can use this to estimate blood alcohol levels.