802.11h is an extension that allows dynamic frequency usage of the 5GHz spectrum.
The story is that in the 802.11n mode the 2.4GHz spectrum is still used for beaconing and control messages, while the 5GHz spectrum is used for data transmission, so the 2.4GHz band is offloaded.
Enabling 802.11h allows stations to dynamically change the data frequency band, which helps in case of interference with neighboring nodes, resulting in a smoother distribution of traffic across the mesh.
Support of this feature varies from adapter to adapter. Newer adapters use 802.11h by default and it is implicitly enabled with no setting. Moderately old adapters have a setting in the adapter properties, so you have to enable it manually. Old adapters may not have 802.11h at all, but updating their drivers might help bring it, because their drivers usually contain wifi chip microcode, which might also have got updated with a newer driver release.