Plain English Meaning
It is a label used inside a pilot weather report to mark which part of the report tells you what kind of aircraft the pilot was flying when they made the observation.
Full Definition
In a Pilot Report (PIREP), /TA is the field identifier that introduces the type of aircraft making the report. It appears as part of the standard PIREP format on aviation weather products, where each element of the report is preceded by a forward-slash code (e.g., /OV for location, /TM for time, /FL for flight level, /TP for aircraft type). Note: in some PIREP formats and FAA publications, aircraft type is coded as /TP rather than /TA; the slash-letter codes identify which piece of information follows.