"Yin" and "yang" refer to the Chinese concepts of opposites bound together within "taiji", their conjunction, which springs from "wuji" which is the unified whole before any division into opposites of "yin" and "yang". While it sounds very philosophical, it ties back into core concepts in martial arts as applied physics, which is why "yin" and "yang", even being Chinese terms, or the equivalent, are omnipresent in many martial arts.

It's about opposites, in practice.

Put simply, if you take a stick, and you push on one end, and you pull on the other end, the stick will rotate. If you push on a person's right shoulder, while pulling on a person's left shoulder, they will spin. If you push on a person's shoulders, while pulling on their hips, they will fall over. This leads to many techniques having a "push-pull" action that is designed to treat that person in some way, somehow, like that stick and twist them, knock them over, sweep them, or otherwise. This can also express as other derives concepts such as levers and torque, tensioning like with a rope or sheet, pivoting, and more. Basically, using opposites in application makes a lot of techniques a whole lot easier.