IPA: /ˌænəˈkruːsɪs/
KK: /ænəˈkruːsɪs/
A musical or poetic term referring to one or more unstressed syllables or notes that come before the main part of a line or phrase.
The song began with an anacrusis, setting the rhythm before the first strong beat.
Anacrusis is formed from "ana-" (meaning up or back) and "krousis" (from Greek "krouein", meaning to strike or to beat). The term refers to a note or notes that are played or sung before the first strong beat of a measure, thus leading up to the main part of the music.
Think of 'up' ('ana-') leading to a 'strike' or 'beat' ('krousis') in music — that's why anacrusis refers to notes that come before the main beat.