In order to know how much of the 3/8 is in the 1/4, you calculate the quotient between 1/4 and 3/8, just as follow:
1/4 ÷ 3/8
you can convert the previous quotient into a multiplication by inverting the second fraction:
1/4 ÷ 3/8 = (1/4) x (8/3) = 8/12 = 2/3
Hence, 2/3 is what 3/8 is in 1/4