A fix that is not guarded is a fix that can silently come back. The guard is the same gate that failed, run on every release, plus a record of the number that passed. The record is what turns a later pass into a known pass rather than an unknown one.
About 1 min of reading
Add the gate and the record
Run the gate
Before you run it, say out loud what it will print. Then open the answer.
Record the number
Before you run it, say out loud what it will print. Then open the answer.
Compare to the previous release
Before you run it, say out loud what it will print. Then open the answer.
Tighten the threshold
Before you run it, say out loud what it will print. Then open the answer.
Comparing releases. Without it, a passing gate tells you the page is under a threshold and nothing else.
If the gate measures a page nobody visits, it guards the wrong thing. Name the page in the budget and keep it the page users load.
The gate is part of the release, not a task someone remembers. A gate that runs only when a person chooses is not a guard.
Store the measured value with the release. A later release that passes at a much worse number is a regression the gate did not catch.
Once a fix lands, set the next threshold near the number you achieved. Budgets move down as the site gets faster, and the record is what tells you the new number.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.