The Phazon Meter is the yellow bar at the top of the screen. Just being on this planet causes this meter to rise. If it fills up, you die.
To prevent it from filling up you need to collect Anti-Phazon Orbs, which are essentially Energy Orbs, but work backwards. You get these orbs by killing enemies.
You can also lower your Phazon Meter by using your Hyper Grapple to send Phazon into enemies or by standing near Phazon Puffers who suck Phazon from you (just don't get hit by their attack).