What is alt-text, how do you do it, why do you do it, what needs doing?
What is it?
Alt-text is the 1500 characters you get to describe an image you post.
How do I do it?
If you're posting a tweet/screenshot of an article than I'd advise using the desktop/Web login to Mastodon. It has a fairly good auto-transcribe feature which requires only minor edits.
For memes, photos and cartoons you're going to have to do it the more difficult way but it's a good habit to get into.
4/T