Plain English Meaning
FM tells the pilot that, starting at this exact day and time, the weather is expected to switch over fairly quickly to the new conditions described next. From that point on, only the new conditions apply.
Full Definition
In a Terminal Aerodrome Forecast (TAF), FM is a change indicator marking a rapid and significant shift in forecast conditions, expected to occur within one hour. It is followed by a six-digit time group (DDHHMM) giving the day, hour, and minute the new conditions begin. Everything stated after an FM group replaces the previous forecast conditions until the next change group or the end of the TAF.
Context Anchor
Seen in TAF weather reports, usually written as an FM group followed by a date and time, such as FM151800.
Derivation
FM is simply the first and last letters of the word 'from,' chosen because TAFs use compact codes to save space in transmission.
Why Pilots Care
It tells the pilot the precise weather is expected to change so they can plan , , or decisions accurately.