You probably know what it means to have wrinkles on a shirt--or even on your face. (If you don't know yet, just wait a few years!)
But "wrinkle" has a different meaning that you might not know about. If you have a plan or a complicated situation, a "wrinkle" is a small detail that makes the plan a little more difficult or complicated.
The Gregorian calendar gives us a leap year every four years. Every four years, we have an extra day in February. You know that, right? But there's a wrinkle. Century years like 1800 and 1900 should be leap years, but they're not. Following the once-every-four-years pattern, 1900 should be a leap year, but it's not. That's a wrinkle with the Gregorian calendar--a small detail that makes things more complicated.
Here's another wrinkle: century years divisible by 400 are leap years! So while 1700, 1800, 1900, and 2100 are not leap years, 1600 and 2000 are.
Any time you have a small detail that complicates a bigger plan or situation, you have a wrinkle.