By high frequency, I mean any sound in the upper teens of the frequency range, say 14k and up. For instance, the highest components of the sound of a cymbal crash will fall into that range. Or the sound of a very sibilant "S" being sung by a singer will fall into that range.

By rolloff, I mean that the encoder will use a filter to remove those high frequencies completely from the music before it encodes it.

Some encoders will roll of the high frequencies in such a way that it's painfully obvious when you play back the MP3 on good equipment. Others will do it better, by choosing a higher cutoff point (for instance, by doing it at 19k instead of at 16k).