The internet slang term “tho” is a contraction or a shorthand version of the English word “though.” It is used to mean “on the contrary” or more simply, “however.” example : “ I can’t really eat spicy food, but I still enjoy Korean rice cakes tho”

As Anais said, it's just a short way of writing "though." It's not slang. It's just a different spelling. It means "though," and it's pronounced the same as "though."