By leaving out the 4th and 7th degrees of either a major scale or a mixolydian scale, the major pentatonic scale is created. As the name suggests, pentatonic scales contain only five different notes. There are many melodies, particularly in folk and gospel music which are derived from the major pentatonic scale. Here is a comparison of the G mixolydian scale and the G major pentatonic scale.
Here is the G major pentatonic scale played over two octaves. Once again, mentally name the scale degrees as you play. Do this until you know which degree you are on as soon as you play it.