Plain English Meaning
An updated airport weather forecast issued early because the original one no longer matches what is happening or what is expected. When you see AMD, treat the new forecast as the one to use and ignore the older version.
Full Definition
A TAF AMD is a Terminal Aerodrome Forecast that has been re-issued before its scheduled update time because conditions at the airport have changed, or are expected to change, in a way that makes the original forecast no longer accurate. The amended forecast replaces the previous TAF for that airport and is identified by the letters AMD inserted after the TAF header.
Context Anchor
Seen in TAF weather reports before a flight, especially when checking current forecast information for departure, destination, or alternate airports.
Derivation
AMD is short for 'amended,' from the Latin emendare, meaning 'to correct' or 'to free from fault.' The label flags that the forecast has been , not just updated on schedule.