39 is a number that comes after 38 and before 40. It is made up of three digits, 3 and 9. When we count things, we can use 39 to represent how many things we have counted, like 39 apples or 39 toys.
If we look at the digits in 39, we can notice that the number 3 comes first and it means we have 3 groups of ten. This can be written as 3x10=30. Then, the digit 9 represents how many ones we have left, which means we have 9 more things than the 3 groups of ten. So, altogether we have 39 things.
Sometimes, people think that 39 is an unlucky number because when you say it in Chinese, it sounds like the words for "painful" or "difficult". But other people just see it as any other number and don't believe in things being unlucky or lucky.