Sometimes a song does not begin on the first beat of the bar. Any notes which come before the first full bar are called lead-in notes (or pick-up notes). When lead-in notes are used, the last bar is also 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 first full bar, after the lead-in notes.
The next song contains first and second endings. The first time you play through the song, play the first ending, (Z), then go back to the beginning. The second time you play through the song, play the second ending (X) instead of the first.
This example incorporates a lead in and first and second endings. It also features the use of quarter rests in bars 2 and 4.