"Fresh air" is air that:

• comes from outside
• is clean
• doesn't smell bad
• is dry and cool

Air that you breathe out in the countryside on a cool, sunny day is the perfect example of "fresh air". But if you're riding in a dirty car with no air conditioning in the summer, then you might call the air from outside the car "fresh", even if it's dirty and warm.

When people want to breathe fresh air, they say "I want to get some fresh air." If they're in a room or car, they might say "Let's get some fresh air in here."