“AiO” stands for “All-in-One” and means the pad has all the absorbency and waterproofing (PUL) materials sewn in, and is simply placed into your underpants as-is, like you would use a disposable pad.

The pad listings include letters in brackets to tell you the absorbency and waterproofing.
“L” = Light.  “R” = Regular.  “H” = Heavy. “w” means Waterproofed with PUL.