Sometimes a song does not begin on the first beat of a bar. Any notes which come before the first full bar are lead-in notes (or pick-up notes). When we use lead-in notes, the last bar is incomplete. The notes in the lead-in and the notes in the last bar add up to one full bar. When you are playing chords, do not strum until the start of the first full bar, after the lead-in notes.
Strum Pattern
Strum Pattern