When something happens per annum, it occurs every year. Your job at the animal shelter might not pay much per annum, but if you love animals that won’t matter too much.
You’ll most often find the phrase per annum describing someone’s salary, a fixed payment, or some other financial detail. If your rent is five hundred dollars per month, you could also say that it’s six thousand dollars per annum. And if an investment earns you a certain amount of money each year, you can talk about the per annum return you get. Per annum means “by the year” in Latin.