French-English slang meaning very cool.
Tres, or often très, comes from the French très, meaning 'very', 'most', or 'very much'. Cool is English. The phrase tres cool is used by both French and English speakers. Saying tres cool is way cooler than saying cool.
If something is exceptionally cool, you might refer to it as tres tres cool. You can add on as many tres's as you need. Variations include tres swank and tres good.
It is pronounced Treh Kool (the S is silent). Chances are that no-one will notice if you pronounce it tres or trez Kool; they probably say it that way themselves. But if you want to be pedantic in your slang, now you know.