In order to compare fractions, you must find the common denominator. The common denominator is essentially the least common multiple of the two denominators. In this case, it is 15 since that is the least common multiple of 3 and 5.
Next you need to multiply the numerator with the number you used to get 15 in the denominator. So for 1/3, you multiply the 1 by 5 since you had previously multiplied the 3 in the denominator by 5. So now your new fraction is 5/15. You do the same thing with 2/5 by multiplying 2 by 3 since you multiplied 5 in the denominator by 3 to get 15. Now your other new fraction is 6/15.
Now that the denominators are the same and the numerators have been changed to that the fractions actually still remain to be the same value, you can simply compare the numerators, 5 in 5/15 and 6 in 6/15. 6 is larger than 5 so therefore 6/15 is larger. 6/15 was originally 2/5 thus 2/5 is larger than 1/3.