The Guardians of Far Madding is a ter'angreal that sits in the Hall of Council's chambers and prohibits channelling in Far Madding. It also detects if somehow a woman or a man could channel in the city e.g. using a Well.
It is made of three large discs a full span across that looked like clouded crystal had been set into the floor and inlaid with a long thin wedge of clear crystal that pointed toward the chamber's center. Metal collar surrounded them, marked off like a compass with small markings looked like numerals. 
The device was almost certainly created at some time during the Breaking, since it was plainly directed at men who could channel. The built-in defence blocks all channeller to be able to study it with the One Power or reproduce it in any form.
The three discs create an area around the city that cuts off channelers from the True Source. The Guardian is similar to a stedding because it stops channelers from embracing the True Source but not destroying their weaves. So a well can be used inside the dome, like when Nynaeve used one to set Aleis Barsalla's crown back on her head with a weave of Air.
Men are cut off about a mile around the city, well beyond the lake, and women at the city limits. The third disc creates an even larger area that allows channelers to be tracked. Each of the disks has within it a crystal wedge which ordinarily points to the center of the Guardian's chamber. When a channeler channels, the wedges all swing to point to him or her. When the Guardian detects saidar, the wedges turn red, and when it detects saidin they turn black. The stones around the three rings are marked with highly detailed points of the compass, allowing the channeler's position relative to the ter'angreal to be plotted on a map.
Similar to an Ogier stedding channelers cannot even sense the True Source and anything channelled against the city simply melted away at the edge of the active area. Even balefire dispersed on hitting that border.  But as with Mat's Foxhead medallion Ter'angreal this protection does not mean that Far Madding is protected also from indirect uses of the One Power, for instance a great boulder thrown with the One Power from outside the perimeter defended by the Guardian could hit the city with devastating effects.
After Rand merged with Lews Therin travels there to meet the Borderlander kings and queens and he mysteriously says that The Guardian blocks the One Power only. He might mean that had he been turned to the Shadow, he could use the True Power of Shai'tan which cannot be blocked by the Guardians.