Loosely put, a channel group is a group of channels which transmit on the same frequency or set of frequencies.
Normally, in every cell, we have the BCCH TRX transmitting on one frequency(usually non-hopping), and some other TRXs hopping on a set of frequencies.
The channels(timeslots) in the BCCH TRX belong to one channel group, usually referred to as CHGR0, because they all on the same frequency. The other channels on the other TRXs( which all hop on the same set of frequencies) also belong to one channel group, referred to as CHGR1. Note that you can have other channel groups other than 0 and 1, eg, you can decide to define a channel group 2 or 3.