The Rock Meter is found in the lower right corner of the screen (Lower left in Guitar Hero World Tour), and measures how well you're doing while playing a song. It is divided into four colored sections: green, yellow, red and blinking red. At the beginning of each song, the Rock Meter's needle is set to the center (except for encore songs, where it starts at middle green). As you correctly play, or "hit," a note, the needle moves one step to the right (towards the green section). Conversely, as you miss a note or overstrum, the needle moves three steps to the left (towards the red section). If the needle reaches the far left of the meter, the player fails the song. However, if you are in blinking red, just barely hanging in there, you can activate Star Power and for every note you hit, the needle will drastically move to the right (green) giving you that extra boost to help you through a song.
In Guitar Hero and Guitar Hero 2, the guitarists play the guitar sadly when the rock meter is red, make a calm looking face when in yellow, and show their usual expressions when the meter is green. Since Guitar Hero 3, facial expressions never change.