Rests - Musical Silence
Rests show you where not to play. A rest means you should be silent which is actually quite hard when playing the guitar. If a string is ringing, you must silence it.
The most effective way to silence a string is with your right hand. Simply touch the string with one of your fingers to stop it ringing. This technique works whatever you are doing with your left hand. You may also use the palm of your hand if you need to silence all the strings.