A full bar is when the 1st finger of your left hand frets all six strings at the same time. In the following piece a full bar is used on the 1st fret in the 13th bar of the music. To finger a bar correctly ensure your left hand thumb is behind the neck and your first finger is straight, inline with the fretwire, as shown in the photo.
The following exercise is in + time, a total of 12 eighth notes to a bar. The symbol CI indicates a full bar at the first fret, C indicating bar, I (Roman numeral) indicating 1. Note: In bars 10 and 14 a half bar is used. In both cases only the 1st and 2nd strings are barred.