AMR stands for Adaptive Multi Rate speech codec. This actually is a set of speech codec modes at different bit rates.
Bitrates are 12.2-FR1; 10.2-FR2; 7.95-FR3/HR1; 7.4-FR4/HR2; 6.7-FR5/HR3; 5.9-FR6/HR4; 5.15-FR7/HR5, and 4.75-FR8/HR6.
AMR FR are 22.8kbps and AMR HR are 11.4kbps. The rest is the channel coding. I meen for example for FR8: speech coding 4.75 and 22.8-4.75=18.05 channel coding. This increases the robustness to channel errors due to bad radio conditions pretty much. In fact they say that the same quality as EFR at 10dB C/I could be achieved with AMR-FR at 4dB C/I.
I hope this helps