When you have 2/5 and 1/2, the common denominator in this case is 5 X 2 = 10.
If you change the two fractions to be represented by the common denominator 10, 2/5=4/10 and 1/2=5/10
You have your answer: 2/5=4/10<5/10=1/2.
If you are more comfortable thinking about slices of pizza, in order to get 2/5 of a pizza, it needs to be cut into 5 equal pieces, to get 1/2, 2 pieces. To get both 2/5 and 1/2 off the same pizza, in equal slices, you will need to cut the pizza into 10 pieces, and take 4 out for 2/5 and 5 out for 1/2.
Common denominator can easily be obtained by multipling the numbers. However, the least common denominator could be way smaller than that. Going back to the pizza case, if we want 1/6 and 1/2 of the same pizza, we don't need to cut the pizza into 6X2=12 slices, 6 slices are enough, because the least common denominator of 2 and 6 is actually 6 itself.
Hope it helps. Good luck!