There are four stages to using Picard to process your audio files:
Retrieving album information from MusicBrainz
This stage identifies the album on MusicBrainz that will provide the information used for tagging the files, and retrieves the metadata from the MusicBrainz database. There are a few different methods available, depending on the information currently available on your system (e.g.: metadata existing in the files, or having the source CD available).
Matching audio files to tracks
This stage is where individual files are matched to specific tracks in the information retrieved from the MusicBrainz database.
Selecting Cover Art
Depending on the option settings, you can change or confirm the cover art to save with a track or album.
Saving the updated audio files
This stage is where Picard updates the matched files with the metadata retrieved in the first stage, based on the settings configured in the Options. This may also include renaming the files and placing them in a different directory.