Answer 1

It calculates remainder...
38%5 == 3
If we divide 38 by 5
38/5
3 is remainder
because 5*7 == 35
and 38 - 35 == 3
So its output will be 3
ANOTHER WAY can be
38 = 5 + 5 + 5 + 5 + 5 + 5 + 5 + 3
here 3 is different and hence the output.

Answer 2

Modulo has been already explained infinite times.
https://www.sololearn.com/Discuss/1862807/how-do-you-explain-the-modulo-operation
https://www.sololearn.com/Discuss/2072362/is-anyone-able-explain-modulus-in-javascript-to-me
https://www.sololearn.com/Discuss/649643/explain-me-please-the-modulus
https://www.sololearn.com/Discuss/1192954/explain-modulo
Please search for same questions before you post something.