A "node" is a stationary point in a vibrating system.
Visualise a skipping rope, or tail rotor drive shaft. Start shaking it at one end, keeping the other held firm. As it vibrates, one part of the rope/shaft won't move - that's the node. Shake it / turn it faster, the node moves.
The lead weights I assume move the nodes in the AS350 cabin to near the seat attachment points - less movement, less bounce in the pilots' step when they get out.