Tremolo is a way of sustaining notes on the mandolin. It involves rapid playing of a note with continuous alternating down and up strokes of the pick, played as fast as possible. Tremolo is indicated by diagonal lines, as shown in the following example. Hold the pick close to the tip and make the rapid movements from the wrist rather than the arm.
This example shows a typical use of tremolo. Once you have control of the technique, try applying it to the longer notes in "El Condor Pasa" and other melodies from earlier in the book - e.g., Londonderry Air. Tremolo takes quite a while to master, so be patient with it.