A tuned mass damper is a damped mass-spring-system that is attached to a vibrating main system.
The behavior of the TMD in the six possible directions and rotations can be adjusted with springs and dampers.
By doing this, the TMD can reduce vibrations in various directions and frequencies with just one mass.
As you see in the attached picture, the right structure vibration is much reduced and decays rapidly than the left structure due to the Tuned Mass Damper effect.