According to lottiefiles.com :
"Lottie is an open source animation file format that’s tiny, high quality, interactive, and can be manipulated at runtime. The top 500 apps on the App store now use Lottie to engage users and enhance conversions."
It's basically an animation format widely used for websites and apps. To export a Lottie file requires a plugin (LottieFiles, or BodyMovin) to use directly with After Effects. The file is a small .json to embed in your project.