I understand it now. It's the relative frequency type of probability used by insurance companies etc.

Take a representative sample, say 100 individuals, of 99 year olds and wait 5 years. If 6 of them die, the relative frequency of death in 99 year olds over a period of 5 years is 6/100 = 6%.

That means any 99 year old now has a 6% probability of dying in the coming 5 years.