CO detectors have a certain range of what is a normal carbon monoxide level and anything outside this normal zone will trigger an alarm. Peak level memory means that the device remembers how high the levels were when it was going off so if you ever need to know that information for any reason, you can take a look at your detector rather than having to go into all sorts of other trouble trying to track down where exactly in your home these dangerous levels occurred. This feature makes life much easier!