Like macros in a word processor or actions in Photoshop or similar image manipulation program, chains can be used for ‘batch processing and effects automation,’ allowing you to apply the same effect to multiple files, thereby speeding up your open-source sound production and reclaiming more of your own time. They’re pretty simple to learn and thoroughly documented here on the Audacity wiki.
There are two related Chain commands in the File Menu called Edit Chains and Apply Chain. Look first at Edit Chains to determine if an existing Chain (Audacity only ships with one) will suit your needs or to create a new chain. Next, use Apply Chain to perform the action on your selected audio.
The two uses of Chains are for:
Batch processing (where many audio files are processed unattended with one or more effects then exported to a new file) Effects automation (where the selected audio in the track or tracks in the current project is subjected to the same prescribed sequence of effects, and optionally, a file exported from the entire audio.)