Two fractions are equivalent when they are equal when reduced to the lowet terms.

Dividing both numerator and denominator by 2 we get $\frac{4}{6} = \frac{2}{3}$

Dividing both numerator and denominator by 5 we get $\frac{10}{15} = \frac{2}{3}$

So, as we can see, $\frac{4}{6}$ is equivalent to $\frac{10}{15}$.