Plain English Meaning
An INS works out where you are by adding up movement from where you started. The sensors that measure that movement aren't perfect, so the calculated position slowly drifts away from the real one. The longer the flight, the bigger the drift.
Full Definition
The small, accumulating inaccuracies in position and velocity output by an Inertial Navigation System (INS), caused by tiny imperfections in its accelerometers and gyros that compound over time. Because an INS calculates position by continuously integrating sensed accelerations from a known starting point, any sensor bias or drift grows steadily into a position error the longer the system runs without an external reference update.