In the movie 'Rounders', what does Mike D. mean when he said, "I have what's known as the wheel. It's got earthy tones, a smooth draw and enough kick to win me the high and the low" and what game was he playing?

First, they were playing poker. A "wheel" in poker is the A-2-3-4-5. It's a straight flush, so it could win high hand, and they also must have been playing high-low split, so it would always win low hand.

https://en.wikipedia.org/wiki/Lowball_(poker)#Wheel
https://en.wikipedia.org/wiki/High-low_split
https://en.wikipedia.org/wiki/Lowball_(poker)

It's a straight, and it could be a straight flush if all of the cards are the same suit. Even a straight could be high hand, but a straight flush would almost certainly be high hand.