The field is the diffuse force surrounding a node. Prior to the destruction of all the nodes in 208 ATM, the field was the source of most mancers power (though some such as Yggur and Malien, were able to draw from older powers). When the nodes were destroyed, all fields were destroyed along with it, stripping most mancers of their power.
More powerful forces, known as the strong forces, were known to surround nodes as well, however, few people ever successfully drew power from them, due to the incredible potency of such forces.
Runcible Nunar discovered how to draw power from the field in 100 ATM, and is often credited with being responsible for the concept of controllers and clankers.