$$2/3$$ is a fraction in simplest form.
$$4/6, 6/9, 8/12, 10/15,$$ and $$12/18$$ are all equivalent to $$2/3,$$ because they can all be simplified to $$2/3.$$
We get these fractions by multiplying numbers to the numerator/denominator.
For example, $$2 \times 2 = 4$$ and $$2 \times 3 = 6;$$ this gives us $$4/6$$. We multiply $$2/3$$ by $$2/2.$$
Similarly we can multiply the numerator and senominator by $$3,4,5,6$$