I don't understand what Annuities and the Ruls of 72 is? Can someone help explain this to me please.

I don't understand what Annuities and the Ruls of 72 is? Can someone help explain this to me please.
I don't see any direct relation but annuities generally and I mean "generally" since there are many different types of annuities for different financial issues, they generally offer a "lifetime" income, an income that one cannot outlive. It's possible to save, save, then live longer than the usual life expectancy and run out of income... annuities offer a lifetime income. I use the rule of 72's to quickly calculate how long it will take for a given amount of money to double in value at a given interest rate... i.e. $1,000.00 deposited into a bank acct pay 6% interest will double in approx 12 years (6X12=72.) or if you earn 12% it only takes about 6 yrs to double.