Malira,
It means the BTS couldn’t send the Paging, because the PCH (air) is congested. Actually, the BTS buffer is full, but it probably means the same thing : the PCH can’t “evacuate” the Paging Cmd in the buffer, so the buffer is full.
Abis Congestion might affect paging performances. You can see the RSL congestion in the Mode “BTS BSC LAPD” (on the left side of RNO/NPO screen), Counter L1_18.