Hello i am not an expert and not specialiesd in telecommunication technology. but what i understand is "a band" is a specific range of frequencies in the radio frequency (RF) spectrum and the use of these bands are regulated to avoid interferences.
when you buy a phone, the specifications will tell you the type of bands it is supporting. If you look at the picture below. This is the spec for Samsung S7 Edge. and it says it supports 4G Band 5 (850)