Most guys think it's when two people are PHYSICALLY attracted to each other, and get MORE attracted as they talk.
Well, WOMEN know exactly what sexual tension and chemistry are, but men have no real idea because they think it's all based on physical looks.
And because most men aren't young, tall, and handsome they don't even ENTERTAIN the concept that they could enjoy creating and amplifying sexual tension and chemistry with attractive women.
Chemistry is a reference to a FEELING... an EMOTION. If the woman feels the emotion, chemistry is present.
Sexual tension is created by flirting, being suggestive, using humor, and demonstrating that you're sexually confident and aware and that you know what's going on, but you're not desperate... so you can lay back and enjoy the sparks and unpredictable story that's unfolding.

ANTICIPATION is a big part of this puzzle. Most men don't see any need for enjoying anticipation... instead thinking they should just go for it and get it done now... but as Russell Crowe found out in A Beautiful Mind, when you sit down at the bar and say "I mean essentially we are talking about fluid exchange, right? So could we go just straight to the sex?" you just get a drink thrown in your face.