Plain English Meaning
It's the row of dots on the side of the HSI that tells you how far above or below the correct descent path you are during an ILS approach. If the pointer sits in the middle, you're on the path. If it's above center, the path is above you and you need to climb a bit; if it's below center, you're high and need to descend.
Full Definition
A vertical scale on the Horizontal Situation Indicator (HSI) marked with reference dots above and below a center index, used with a moving pointer to show the aircraft's vertical position relative to the ILS glideslope. The center represents being exactly on the glideslope; each dot represents a fixed angular displacement above or below it.
Context Anchor
Seen on an HSI during an that provides to the .