Yes, concentration is a way of optimizing the number of abis timeslots required for signalling.
By using concentration, the signalling(BCCH, SDCCH, FACCH, SACCH, etc) for a number of TRXs are sent on one abis timeslot. If you’re using a concentration factor of 4 (for eg), it means that the signalling information for 4 TRXs are ‘concentrated’ onto one abis timeslot.
Of course, it has drawbacks. If your concentration factor is too high, some of your signalling information can be lost, resulting in poor performance in your cells.