A chronological record of the document being built up over time, with timestamps and named editors, stored on Google's servers.
What it does not
It shows the document changing, not fingers on keys. A determined person with edit access could paste into a document over several sittings, though the pattern of large pastes among small edits is visible to anyone who looks.
How to get it
File, then Version history, then See version history. Expand the dated list, and use the three-dot menu to name versions and to copy a link to a specific version.
How long it survives
For the life of the file, and it survives being shared, commented on and exported because it lives server-side rather than in the file.
Why it matters
It is the strongest single artefact in almost every dispute because you did not create it and you cannot alter it. A record held by a third party is worth more than anything you can produce yourself.
Check the evidence in your own document
The analyzer reads the process record inside a .docx and reports each signal separately, with its limits stated.