Anacrusis is one or more unstressed syllables that come before the first regular metrical beat of a line. Poets use it as a light pickup into the rhythm, so the line begins naturally even though the formal meter starts slightly after the first syllable.
It is sometimes called an initial inversion or pickup, but the key feature is that the opening syllable(s) sit outside the line's main metrical pattern.
Example
Before the dawn, I heard the sparrow sing
and woke to silver stirring in the grass.
In the Star Spangled Banner, the word "Oh" in the first line is an anacrusis.