Spectral Band Replication (SBR) is a technology used in audio coding to increase the efficiency of audio coding at lower bit rates. At lower bit rates an audio coder will not have enough bits available to encode the higher frequencies. SBR is a technology to recreate the higher frequencies out of the encoded lower frequencies together with some additional information. SBR uses the fact that the higher frequencies are usually harmonics of the lower frequencies.
SBR is a technology that is used in association with an audio coder such as MP3 (then called MP3PRO) or AAC (then called AACplus or HE-AAC). The basic audio coder is used to encode the lower frequencies and SBR is used to add some information which can be used by the decoder to rebuilt the higher frequency components out of the lower frequencies together with the additional information.
SBR is usefull in situations where the bitrate is too low for the underlying audio coder to encode the full bandwidth sufficiently well. SBR is e.g. used in the DRM standard for digital radio.