text alternative
A text alternative is a sentence written into the document to stand in for a picture, read out to somebody who cannot see it.
In more detail
It is not a caption. A caption is printed under the picture and everybody reads it; a text alternative is never shown, and only software reading the page aloud says it.
The useful one says what the picture shows, not what it is. A photograph of a shop front, a graph of sales falling by a fifth over the year, a signature in ink — never simply a photograph, a graph, an image.
What it does on a real document
Each picture in the structure tree carries its own sentence. A picture that carries no meaning — a rule, a corner ornament, a background — takes no sentence and is marked as decoration where the document says so, so that nothing reads it out — that mark is asked for too, never worked out from the picture itself.
No program can write it. What a picture means is known to whoever put it there, which is why the accessibility standard asks for it and no engine can supply it.
Where it comes from, and why
The idea is older than the PDF: it came from the web, where a picture has carried a written stand-in since the early nineteen nineties.
The accessibility standard for PDF carried it over, and the European rules on what public bodies publish are what turned it from good practice into an obligation.
See it on a finished document
A list whose pictures speak too The page that goes further on this word