Keyi is a common everyday Mandarin word, used in conversation to say that something is allowed, possible or fine, and the short form heard here has become a handy reaction sound online. Because the clip is under half a second, it travels well in short-form videos, memes and messaging, where a fast yes reads better than a spoken explanation. Creators tend to use it as a universal thumbs-up, even for viewers who do not speak Mandarin, since the tone alone signals agreement, permission and easy confidence.