Plain English Meaning
If the destination's forecast weather, within an hour either side of your arrival, shows the cloud base below 2,000 feet or visibility under 3 miles, you must list a backup airport on your IFR flight plan.
Full Definition
A regulatory shorthand for the IFR alternate airport requirement under 14 CFR 91.169. An alternate airport must be filed on an IFR flight plan if, for at least 1 hour before to 1 hour after the estimated time of arrival at the destination, the forecast weather is less than 2,000 feet ceiling and/or less than 3 statute miles visibility.
Context Anchor
Seen during IFR preflight planning when deciding whether the flight plan must include an alternate airport.