Kun is one of forms of honorific treatment that exists in Japan, used mainly to designate those who are young and male.
the suffix kun it can still be used when a higher-ranking individual is talking to a lower-ranking individual, but only if there is a certain degree of intimacy and friendship between the two.
This is an honorary title also of respect, however with more "softness" and proximity than the typical "san" and "sama”.
Learn more about the meaning of san and sama.
In the West, these forms of treatment have gained popularity thanks to anime and manga (comic books typical of the East, namely Japanese), in which characters use these suffixes according to the level of relationship they have with others characters.
Learn more about the meaning of mango.
For example, in the stories of the ninja Naruto, his close friends often call him Naruto-kun, indicating that there is a degree of intimacy, affection and friendship in these relationships.
See also the meaning of Kung Fu.