One year — that is, exactly one cycle of the Earth's seasons — is approximately 365¼ days long. So if our calendars only counted 365 days in every year, for every four years we'd fall one day behind. Our seasons would shift; after many years, North America would receive snow in June and summer heat in December.
Unfortunately, a year is not exactly 365¼ days long. In reality, a better estimate for the average length of a year is 365.2425 days. Because of this, the established rules for "leap days" in our calendars are a bit more complicated than simply adding one extra day (February 29) every four years.
I wonder: Do we have the best possible system for adjusting to a 365.2425-day year? To answer that question, assuming you're not already familiar with all the details of our system, I would recommend working out a system of adjustments on your own first, and then checking to see if you come up with the same rules.