The green circle is the zombie. The purple circle is you. Click to shoot the zombies and don't let them get you. Use the arrow keys to move. If your health reaches 0, you're out. After 45 seconds, a faster zombie type starts coming. Also, to keep your device from lagging too much, there is a zombie limit of 15. So you'd better be killin'!
Credit to @EagleRay09 for the music block thingy, but credit to Thousand Foot Krutch for the song. Change Log: Feb. 15, 2020 - V 1.1 - Now shows time alive after you die. Dec. 23, 2019 - V 1.0 - Released and bugs fixed.