Complex sentence: In poetry, anacrusis is the lead-in syllables, collectively, that precede the first full measure, while, similarly, in music, it is the note or notes (even a phrase) which precede the first downbeat in a group.
Split into simpler sentences: In poetry, anacrusis is the lead-in syllables, collectively, that precede the first full measure. Similarly, in music, it is the note or notes (even a phrase) which precede the first downbeat in a group.