Now I have heard so much stuff listening to various airports here on Live ATC (love this site by the way!), but I have never understood what these 2 things are:
1. Localizer
2. Altimeter
1) To put it incredibly simply, imagine a straightline narrow radio beam that points down the centerline of the runway. The radio in the airplane can tell which side of this beam it is on, and tells the pilot how far left and right he is.
2) The altimeter in the airplane measures the outside air pressure. The higher you go, the less air pressure there is, lower you go, greater air pressure. This wold be very simple, except mother nature changes the baseline outside air pressure with weather (high pressure systems and low pressure systems). The altimeter setting that ATC and ATIS and ASOS gives is what the outside baseline air pressure is. In North America we use a unit of an inch of mercury in a barometer. Standard is 29.92 inches of mercury. The average high pressure system would raise that to around 30.30" and the average low would bring it down to around 29.60", but it goes higher and lower. The pilot just dials in what the controller tells him the air pressure is, and the instrument reads correctly.