I dont under stand what 5/8 - 2/4

To begin you have to find a common denominator between the two fractions. In this case that number is 8 because 4 is a factor of 8.

2/4 * 2 is basically 2*2 over 4*2 = 4/8

From there simply subtract the numerators

5/8 - 4/8 = 1/8

So 1/8 will your final answer.