Plain English Meaning
Asking for an IFR flight plan to be set up while you are already flying, instead of filing it before you took off.
Full Definition
The procedure of submitting an IFR flight plan to ATC while already airborne, typically by radio to the nearest Flight Service Station or directly to the controlling ATC facility, rather than filing on the ground before departure.
Context Anchor
Seen in flight planning and instrument flying discussions when a pilot needs to start or change planned instrument flight after departure.
Derivation
“File” originally meant to place something in an official record or organized set of papers. In , filing a means putting your planned on record with the . “In flight” simply means this happens .