HE-AAC (High-Efficiency AAC, also AAC+ or aacPlus) extends AAC-LC for low bitrates by adding Spectral Band Replication (SBR). The encoder codes only the lower part of the spectrum with AAC-LC and sends a compact set of parameters that let the decoder regenerate the high frequencies, so you get usable full-bandwidth audio at 32-64 kbps where plain LC would sound dull and filtered. That efficiency made it the codec of choice for digital radio, low-bitrate streaming, and mobile. The tradeoffs are occasional 'watery' or smeared highs from the SBR reconstruction and slightly heavier decode, but support is near-universal today.
