How does inertia affect a person who is not wearing a seatbelt during a collision?
They will 'launch' forward!
Newton's 1st law states that "an object in motion tends to stay in motion". That's inertia! So when a car that is moving comes suddenly to a stop, all of the objects (including people) inside the car 'want' to stay in motion. Seatbelts work by literally holding you back, and thus make sure you don't get thrown forward from your seat. If a person is not wearing a seatbelt, when the car comes to a sudden stop, that person will fly forward and get injured!