"tho" is a piece of slang, popular on the internet, and of recent coinage. But "tho" is not really slang as much as it is a repurposing of a conjunction into an exclamation.
"Tho" is short for "though", which is short for "although". It is used at the end of a sentence or statement to bring emphasis to something noteworthy or unusual.
That Mercedes-Benz tho!
Those shoes tho!
The final scene of "Titantic" tho!
"Although", as a conjunction, is often used to draw emphasis to a statement that is important, but goes against the main point of the sentence. For example:
Although their best player scored 35 points, the basketball team still lost.
Here, the main purpose of the sentence is that the team lost, the "although" brings attention to the statement that their best player scored well. Through a bit of mutation and shortening, this becomes:
Those 35 points tho!
Although here the conjunction doesn't suggest a contrasting situation as well.
Whether use of "tho" as an exclamation will continue and become a common part of speech, or even a regular part of slang, or whether it will just be used to date internet posts to the middle of the teens, remains to be seen. I initially thought it was silly, but I've grown to actually see some meaning in its use.
That internet slang, tho!