I saw a tv show called impractical jokers and I cant understand this joke
Make it rain: An idiom meaning "To throw or drop dollar bills in a show of wealth."
Hoe: A derogatory term/ A slang for a promiscuous female.
"Them" or as you wrote, "dem" is simply used to replace "those" when used in slangs.
So, "I gotta make it rain on dem hoes" means that he had to drop dollar bills on a group of females that he considered to be promiscuous females.