They contain the glidescope equipment for the International Landing System (ILS). I could write a summary but there's a fuller explanation here:
http://home.sprynet.com/~jayschne/Ils2.htm
Painted very bold red/white chequerboard pattern so they stand out in low visibility. They're right alongside where planes touch down and it's bad news for all concerned if the plane hits the equipment.
Some countries also paint radio transmission towers near airports in the red/white pattern.