ZA,
Very briefly, a teetering rotor system has two, and only two, blades attached
to a rigid hub bar, which 'teeters' back and forth on a pivot bolt.
It is much simpler than a fully articulated rotor head.
This is as applied to gyros only, which have fixed pitch blades.
Helicopters incorporate collective pitch into a similar arrangement, but I'm not
up to speed on the detail.
The FAA has a downloadable Rotorcraft Handbook which explains it in reasonable
detail, and is quite readable.
Cheers.