Going back
Every build is a version. Restoring does not delete what came after it — but it does immediately replace what is live.
Every successful build creates a version with a short description. In the editor, under “Behind the scenes” → “History”, the last thirty versions are listed, newest at the top.
What restoring does
Restoring points at an older version — it does not delete the versions after it, and they stay in the list. It also replaces the live version, so anybody who opens the address sees the older one straight away.
The version you are on is marked “current” and has no restore button — there is nowhere to go back to from it. Every other version has one, newer ones included, so after going back you can always go forward again.
What comes back and what does not
Restoring brings back the code and the table structure. Records added in the meantime stay — they are your users' data, not part of a version.
To take records back there is a separate tool: rolling a table back to a point in time, on the “Data” screen.