Double tapping a TikTok does a number of things, depending on what you have tapped. If you have tapped on someone's profile picture, it will take you to their profile. If you have tapped on a video, it will start playing the video. If you have tapped on the share button, it will allow you to share the video with your followers. It also allows you to watch your video in full-screen mode. It also releases heart rows on the video that show you have liked the video. That is why, Tiktokers always encourage you to doubt tap on their videos.