After Effects Compositions
To work on files in After Effects you need to place them in a composition, often referred to simply as a comp. A composition is a container that stores layers of video, audio, text, etc. Compositions are essentially independent timelines, so you can think of each composition as being a separate movie project.
You can have multiple compositions in the same project. Compositions are listed with other assets in the project panel. You can also create nested compositions, i.e. one comp inside another.
To create a new composition, select Composition > New Composition from the main menu, or click the Create New Composition icon (pictured right).