d% means roll percentile. This is normally done with two 10 sided dice, one for the tens column and one for the ones. Rolling 00 is considered 100
So you have 01-50 and 51-00; rolling between one of those means you miss and the other means you hit. Most DMs I've met ask the player "high or low" to determine which way they want it before the roll. As the miss chance is 50% a percentile roll is pretty much irrelevant as you can roll pretty much anything or flip a coin to get the result. Percentile is necessary for some things, including specific spells like Confusion or Teleport to determine effects.
Does that answer your question or did I misunderstand you?