In this video, the teacher explains how to compare fractions by creating common denominators. They use the example of comparing the fractions 2/3, 1/2, and 4/5. The teacher demonstrates how to find the least common multiple of the denominators and then multiply the numerators and denominators accordingly to create fractions with equal denominators. Finally, they show how to compare the fractions by looking at the numerators. This lesson provides a clear and step-by-step approach to comparing fractions using common denominators.