You import source files into a project as the basis for footage items and use them as sources for layers. The same file can be the source for multiple footage items, each with its own interpretation settings. Each footage item can be used as the source for one or more layers. You work with collections of layers in a composition.
You primarily work with footage items in the Project panel. You can use the Footage panel to evaluate footage and perform simple editing tasks, such as trimming the duration of a footage item.
You can import many different kinds of files, collections of files, or components of files as sources for individual footage items, including moving image files, still-image files, still-image sequences, and audio files. You can even create footage items yourself within After Effects, such as solids and precompositions. You can import footage items into a project at any time.
When you import files, After Effects does not copy the image data itself into your project but creates a reference link to the source of the footage item, which keeps project files relatively small.
If you delete, rename, or move an imported source file, you break the reference link to that file. When a link is broken, the name of the source file appears in italics in the Project panel, and the File Path column lists it as missing. If the footage item is available, you can reestablish the link—usually just by double-clicking the item and selecting the file again.