glidescope

A glideslope is part of a precision approach (ILS). It gives you information on your attitude indicator (usually on the side depicted by a carrot looking marker) on wher you should be during the descent to the airfield. If the indicator is above the horizon, you are too low. If the indicator is below the horizon, you are too high. If it is in line with the horizon on the attitude indicator, you are "on glide path" and should follow that glideslope to your MDA (minimum descent altitude) until you either see the runway or hit the missed approach point.