Anything monthly happens once a month. A monthly paycheck will enable you to pay your monthly rent. Whew!
Monthly works well as both an adjective and an adverb, describing anything that occurs every month. You might get a monthly paycheck or make monthly phone calls to your grandmother. Or a freelance writer might write a couple of articles monthly and travel monthly to a nearby amusement park. A monthly can also be a magazine that's published once a month.