Lottie is an open-source animation file format that allows designers to create and use animations in mobile and web applications. It was developed by Airbnb and is based on the JSON format, making it lightweight and easy to integrate. Lottie animations can be created using design tools like Adobe After Effects and exported using the Bodymovin plugin. Lottie files are scalable and can be rendered at any size without losing quality, which makes them ideal for responsive design. They support vector graphics and can include various effects, such as color changes and transformations, enhancing user experience in applications and websites.