An ID3 tag is how you add metadata like a cover image, title, and author to an audio file. This is not required but is recommended.
Lots of podcasters add this to their files, and most do it by adding the file to their iTunes library, then right-clicking on it, choosing "get info," and filling out the data. You also have the option to add these tags through DAWs like Adobe Audition.
Here's a pretty good explanation:
https://www.prestopod.com/how-to-add-id3-tags-to-your-podcast-mp3-file/
Please note that if you are uploading your audio to Dovetail in multiple segments, you only need to add ID3 tags to the first segment you upload.