"Annuities" are tantamount to "maintenance fees." Once a patent issues, you will periodically have to pay maintenance fees/annuities to a national agency to prevent your issued patent from going abandoned.
In the United States, the Patent and Trademark Office refers to these as "maintenance fees." However, the intellectual property offices of many foreign countries refer to these fees as "annuities." The concept is essentially the same. Note, however, that each country has their own separate legal requirements in terms of the timing of your fee payments, the required number of fee payments you must make, and the amount you will owe with each fee payment.
In the United States, the Patent and Trademark Office refers to these as "maintenance fees." However, the intellectual property offices of many foreign countries refer to these fees as "annuities." The concept is essentially the same. Note, however, that each country has their own separate legal requirements in terms of the timing of your fee payments, the required number of fee payments you must make, and the amount you will owe with each fee payment.