It's synonymous with quitting
A "walk-off" in ITG is almost the same as failing, except it's worse. It is the equivalent of quitting out of a song intentionally because some part of your performance irks you and it is no longer worth the time and the effort to finish the song.
This primarily happens when a player has too many excellents of other non-fantastic judgments and they are no longer able to beat their personal best score or take a high score on a machine.
Walking-off is only done by the highest level players, and generally speaking they will re-attempt the song from the beginning, hopefully not repeating the performance.
Walk-offs are a deceptively critical issue for many players looking to improve, and it can have detrimental effects that develop roots deeper into the subconscious that can affect your ability to play.
It has wider effects you may not be aware of
The biggest effect with walk-offs is that a player loses the ability to push themselves when things get difficult. A player who develops a habit of walking off will always quit when the song gets tough. As soon as this starts to happen, the player will begin to mentally block themselves. The prevailing thought in their head suddenly becomes an overwhelming desire to stop playing, and the player will constantly repeat this to themselves until eventually they give in.
For players looking to push themselves and improve, this is absolute kryptonite for that and the total opposite of the desired effect. It can be difficult to shake a walk-off habit because it chooses the path of least resistance. Persevering with the song means continued physical and mental suffering, and people with low resolve will easily fall victim to it.
I've seen many friends walk off on songs that they did not need to. They still had plenty of energy and push left and could have easily finished the song, but they had convinced themselves almost from the very beginning of the song that they would walk off when it got tough. Some players even pre-empt their failure by attempting to predict where they'll fail, and this more often than not manifests in a failure at the predicted point in the song.
It is hard to shake this habit, but the only way you can do so is to remind yourself of the gains you'll receive from finishing the song, both the obvious and the not-so-obvious, and continuing to tell yourself that every song is