ILS systems work by sending two beams able to be picked up by aircrafts radios and instrumentation that guide the aircraft down to a safe landing spot in the runway one guides you down on the altitude to the runway in a sort of slope (that’s why it’s called a GLIDESLOPE) and the other guides you into the centreline of a runway from the moment that you hear a pilot say LOCALIZER ALIVE the system is guiding him into the extended centreline of the runway. This is done by a needle on the bottom of the horizontal indicator on the PFD for modern, electronic commercial airliners. And for others it’s done by mechanical gauges.
The GLIDESLOPE works with the same principle, except it shows that needle on the vertical side of the artificial horizon. If you’re to low the needle will tell you by moving up, telling you to climb and if you’re too high it will move down telling you to descend. Aircraft capable of auto land can use the autopilot to interpret this information and make much better adjustments than the pilots can. This is why the auto land is only used in certain conditions that the pilots couldn’t control the plane safely in. I.E- low visibility.