They reduce to the same fraction. That means that the numerator and denominator have a common factor which can be cancelled out.
For example, 6/9, 10/15 and 2/3 are equivalent because 6/9=2×3/(3×3), the common factor is 3; 10/15=2×5/(3×5), the common factor is 5. 3/3=1=5/5 so each fraction reduces to 2/3.