Its the same as damn, but you pronounce the sound of the "Y". You usually say it after something dramatic happens or when some gnarly stuff goes down. The word "son" is put after it lots of times so it makes it sound more buttery.
*some dude dumps his girlfriend*
random on-looker" DAYUMN SON!
*a rapper throws down a sick rhyme"
everyone: DAYUMN NIGGAZ!