ILS stands for Instrument Landing System.

It is used when landing in weather which obscures the view of the runway.

Radio transmitters near the runway broadcast two signals: the localizer which gives the pilot information on whether they are left or right of the runway centerline, and the glideslope, which tells the pilot if they are above or below the optimum slanted approach path to the runway. In addition, there are marker beacons (outer, middle, inner) which give the pilot some indication of how far away from the runway they are.

It's much more complex than this, but you get the general idea...

Cheers,

birdstrike