Plain English Meaning
The behind-the-scenes software that takes a published flight procedure and converts it into the coded form an aircraft's navigation computer can actually fly. Pilots never touch a data packer directly -- it's the tool that turns charted procedures into the data loaded into the FMS.
Full Definition
Software tools used by avionics manufacturers and navigation database providers to encode published instrument procedures into the compact, standardized format that an aircraft's Flight Management System (FMS) can read and fly. Data packers translate the procedure as designed (waypoints, altitudes, course changes, terminators) into ARINC 424 path and terminator (path-terminator) leg codes that define how the FMS should each .