A half bar is when the first finger of your left hand frets the first three strings at the same time. In the following piece a half bar is used on the 5th fret in the 5th and 9th bars of the music. To finger a half bar correctly ensure your left hand thumb is behind the neck and your first finger is almost straight, as shown in the photo. Study the following diagram. A bar is indicated on a diagram by a thick line.
The notes you will be holding are the notes C on the 3rd string (same note as the C on the 1st fret of the 2nd string), E on the 2nd string (same note as the open 1st string) and the A note on the 1st string. The symbol 1/2CV above the music means half bar at the 5th fret, C indicates bar and V (Roman numeral) indicates 5. Sometimes a bracket is also added to the notation to indicate a bar.