There are several important left hand techniques which can make your playing sound more interesting and expressive, as well as enabling you to play things which are not possible without these techniques. The first of these involves flattening the first finger across several strings and pressing down behind a fret as shown in the following photo. This is called making a bar with the finger (similar to a guitarist playing bar chords). When playing the following example, keep the first finger bar down to play all notes at the 5th fret.
Use first finger as a bar across strings
Example 181 combines this technique with finger changes on different frets.