Plain English Meaning
Two small arrows on the HSI that tell you whether you are above, below, or on the correct descent path during an instrument approach. Center the arrows and you are on the slope.
Full Definition
Two indicator arrows on the side of a Horizontal Situation Indicator (HSI) that show the aircraft's position relative to the glideslope during an ILS approach. When both pointers are centered on the reference marks, the aircraft is on the correct glide path; if the pointers are above center, the aircraft is below the glideslope, and if below center, the aircraft is above the glideslope.
Context Anchor
Seen on an HSI during an instrument approach that provides glideslope guidance, especially while tracking the descent path to the runway.