Nitrogen and air are two of the most common gasses to fill inflatable tires. Nitrogen molecules are larger than air molecules, making it harder to leak over time. With air in your tires, you may lose pressure in your tires and need to top them off more often.
Nitrogen-filled tires are 74% more effective at maintaining proper tire pressure. You can safely top off your nitrogen tires with air, and it won't harm them. It will dilute the purity of the nitrogen in your tires, but it's better to keep your tires properly inflated.
Nitrogen in your tires has some definite perks, but there are also downsides. The costs to fill tires with nitrogen are substantially more expensive than air. Nitrogen isn't as readily available as air and requires a visit to a tire shop to top off.