What is Tri-band 2G CDMA (800 (CDMA over ESMR), 850, 1900 MHz) in Iphone 5 CDMA model?
It means its compatible with 3 bandwidth for 2G : 


800 MHz (CDMA over ESMR)
850 MHz (GSM)
1900 MHz (GSM)


800/1900 is mostly used in the USA and Canada. It's quite usefull to use 2G since 3G has not a coverage as wide as 2G.