An HRTF (Head-Related Transfer Function) describes how a sound from a particular direction is filtered by your head, torso, and the shape of your outer ears before it reaches each eardrum. Those filters encode the cues the brain uses to localize sound — interaural time and level differences, plus the spectral notches that tell up from down and front from back. Binaural rendering applies a pair of HRTFs to place a virtual source convincingly over plain headphones. Because everyone's anatomy differs, generic HRTFs work but personalized ones localize better, which is why platforms now scan ear shape or let you pick a profile. HRTFs are the math under almost all headphone spatial audio.
