When you shave the hair on your face and neck, you remove both your hair and also the top layer of your skin.
As your hair starts to grow back, it can curl and turn inwards after shaving. As the top layer of skin begins to form again, it can trap the hair, causing it to be ingrown - meaning that you’re now left with a bump on your skin, and all of the irritation, redness and dryness that that entails.
It is in this way that ingrown hairs cause razor bumps to appear. Razor bumps are more common in people with curly hair, owing to the fact that their hair is more likely to curl back into the skin.