reading order
The reading order is the order a document is meant to be read in, which is not always the order its marks were drawn in.
In more detail
A page records where the ink went, one instruction after another. That sequence is whatever the program that wrote the file happened to produce: a footer may be drawn before the body, a caption before the picture, the right-hand column before the left.
For somebody looking at the page it makes no difference — the eye sees the layout. For anything reading the file rather than looking at it, the drawing order is all there is, unless the document states the order it means.
What it does on a real document
On a page in two columns the difference is plain: read in the wrong order, the first line of the left column is followed by the first line of the right, and the text turns to nonsense. The same happens on an invoice whose totals were drawn before its lines.
The order lives in the structure tree: the branches are walked in the order they were written, and that is the order the page is read in. A running head, a folio or a rule is marked as decoration where the document says so, and then nothing reads it out at all — the mark is asked for, never assumed from where the ink fell.
Where it comes from, and why
It came in with the tagged PDF, and it is the reason tagging exists at all: everything else a tag says can be guessed at from the layout, and the order cannot.
The accessibility standard makes it a requirement rather than a courtesy, and the European rules on what public bodies publish point at that standard.
See it on a finished document
A document that can be read aloud The page that goes further on this word