Bluetooth audio codecs determine how much audio data fits over the wireless link. From worst to best: SBC < AAC < aptX < aptX HD < LDAC < aptX Lossless.
Quick reference (typical max bitrate): SBC: 328 kbps — universal fallback, average quality. AAC: 250 kbps — Apple ecosystem standard, sounds good despite lower bitrate. aptX: 352 kbps — Android baseline. aptX HD: 576 kbps — Hi-Res adjacent. LDAC 990: 990 kbps — best lossy. aptX Lossless: 1200 kbps — true CD-quality.
Both source (phone) and sink (headphones) must support the codec. Many headphones list multiple — actual codec used negotiated automatically based on what both support.