A high FCS error rate indicates wireless link interference in the area.
Frames that are transmitted by APs are susceptible to interference from other devices with radios operating in the same frequencies range (same channel), or electromagnetic interference from electronic devices such as power cables in the office.
The 802.11 MAC layer uses the Frame Check Sequence (FCS) field to determine if errors have occurred during the transmission.
Each MAC layer frame has a FCS field that is used to store a checksum. The checksum is added at the source AP, and verified at the destination.
If the FCS checksum included in the frame doesn't match the recalculated number then an error has occurred during the transmission, the frame is discarded and the destination host requests it to be resent.
This can effectively reduce the bandwidth and throughput in the network.
So when you see a high FCS error rate on the AP Dot11 counters, this could indicate that the APs are experiencing a high level of link interference and the clients are getting less bandwidth and throughput due to MAC layer frame retransmissions.