So what does it mean when someone says "he got off on a technicality"?

There have been plenty of cases where exactly that happened.

There's the spirit of the law and the letter of the law, and many are persuaded that when someone hasn't violated the letter of the law, they haven't violated the law.

Judges, lawyers, and juries will vary on this, but in many cases the letter of the law will carry the day, and people will in fact "get off on a technicality".