TCP congestion control works by maintaining a window of bytes that have not yet been acknowledged.
Thus TCP's window (and hence throughput) is a function of the losses observed in the network and the round-trip time experienced by the flow.
The idea behind TFRC is to measure the loss probability and round-trip time and to use these as the parameters to a model of TCP throughput.
The expected throughput from this model is then used to directly drive the transmit rate of a TFRC flow.
RFC 5348 - TCP Friendly Rate Control (TFRC): Protocol Specification