Year-over-year (YoY) compares a number, such as revenue, earnings, or cash flow, to the same stretch of time one year earlier. It is the main way growth gets reported in financial statements.
For yearly statements, this is just this year against last year. For quarterly statements, it is usually this quarter against the same quarter a year ago. The reason is that many businesses are seasonal, like retail, farming, and tax preparation. Comparing the fourth quarter to the third quarter of the same year would mislead you, but comparing this year's fourth quarter to last year's fourth quarter is a fair match.