Timing Belt

I was not sure about this part of the car until a friend of mine tried to explain its utility yesterday. So I was careless driving and did not bother if it was working or not and he was in the car with me. Then after we had reached the destination he asked me why I am not changing the timing belt . I said that It did not matter since I did not know much about that. Then he told me that if I did not change it I might get into some accident. So to prevent it I must replace it with a new one.