A node could be used telepathically to open a door between dimensions, crossing the dimensional divide. However, a node activator was required to use both nodes together, with a Time Lord slaved into the device.
They were locked in a time bubble in a vault on Tersurus, but the Master retrieved them and planted one on Earth and the other on the Tolians' other-dimensional planet, where they began absorbing interdimensional energy and growing. The Seventh Doctor destroyed one by dropping it on the ground, implying they were fragile, but was mysterious as to whether he destroyed the second node. (AUDIO: UNIT: Dominion)