mii-tool and "flow control"?

Greetings:

Could someone explain what "flow-control" means? When we run the "mii-
tool" command on a few of our machines some report with flow-control
and some don't. Same network, same OS, patch level and hardware
configuration. Just want to understand what this might mean.

eth0: negotiated 100baseTx-FD, link ok

eth0: negotiated 100baseTx-FD flow-control, link ok

Thanks for any help.
Tom