The solar year is 365 days. The lunar year is 354 days. Every year the solar year is 11 days longer than the lunar year. A lunar leap year, adding an extra Adar, pushes the lunar year forward by 19 days (compared to the solar year, because the leap lunar year is 374 days) for the year. By having 7 leap years in a 19-year span, we add 19 days * 7 leap years = 133. Meanwhile, we lose 11 days * 12 non-leap years = 132. The difference of 133 and 132 is usually made up by either the solar leap year of one extra day every 4 years in the form of February 29th, or by having the lunar leap year month of Adar have either one or two days Rosh Chodesh.
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