through bore slip-ring torque rating
The torque rating would be the torque required to overcome the friction between the brushes and the slip-rings. It represents friction that will be added to the friction of the friction of the rotating mechanism in which it is installed. The specification is listed as less-than or equal to 0.88 gm (for reference) because it will vary in actual use but should not exceed the stated figure. If the given limit is large in comparison with the friction in the rest of the system, that could be a problem, otherwise it may not be of much importance.

Grams-force - meters is not a standard unit of measurement, but it is used occasionally. 1 gram meter = 0.009807 newton meter.