The airbag warning light comes on when there's a fault with the airbag system, or sometimes the seatbelt pretensioners that work alongside it.
This isn't a cosmetic warning. If it stays on, there's a real chance the airbags won't deploy correctly if you're in a collision - and there's no way to tell from the light alone which part of the system is affected.
Of all the warning lights, this is one to get checked properly rather than wait out, even though the car will otherwise drive completely normally.