Star Power is a power in the Guitar Hero Franchise and the Rock Band Franchise.
You first get Star Power by hitting certain notes that have stars around them. In order to get a quarter of star power, you need to hit every star power note in a starred section (miss one and the notes turn back to normal). If you hit every star power note, you get a quarter of star power, you need at least half a bar of star power to activate. Once activated (you tilt the guitar up) all notes turn blue, you crowd meter will go up faster, and your multiplier will double. So 2x = 4x and 4x = 8x, 8x is the maximum to multiplier. Where you will use your star power in a song will affect your score. Using it when chords are being played will get you more points than using it when single notes are being played.
In the first 3 Guitar Hero games and the PS2-only spinoff Rocks the 80s, when using star power, you cannot obtain more star power. This was changed starting with Guitar Hero: World Tour.
In Rock Band, it's called Overdrive, but it's essentially the same thing. You still tilt the guitar up to activate it, but for drum there are certain sections where you can play a fill. You end that fill with the green pad and that activates star power. For vocalists, its works the same way as drums. At certain sections, you can shout into the microphone and activate it.
When using Overdrive, you can get more Overdrive along the way.