Brian,
If you look in the FS2004 flying lessons it is all explained there in detail
But the short version is that two radio beams are 'fired' out from the runway which instruments in your plane pick up - one gives guidance if your plane is to the left or right of the correct course to land on the runway and the other tells you if you are above or below the correct descent path to land on the runway.
The HSI (or Horizontal Situation Indicator) on your instrument panel shows by pointers if you need to fly to the right or left or up or down and if you keep these pointers centred in the instrument you will approach the runway correctly. When you can see the runway you then land manually or if your plane is VERY expensive it might land automatically for you
Basically it helps you find the runway in poor visibility.
Like I say it's properly explained with diagrams in the FS2004 flying lessons...
