Stands for All The Gear All The Time, means you love riding, and you want to keep on riding regardless.
The gear list must be motorcycling specific gloves, jacket, pants, boots and a full-face helmet. Open-face does not count (leave your face on the road and so long for ATGATT, buddy). Jeans do not count, fall down going 20mph, and voila! you have nice big fleshless spots on your knees!
Remember, without your gear you have only one chance of accident. Crash once, and you're done for.