"Robot ears" refer to headphone (or headset)-like accessories or adornments that may optionally have antennae or blades protruding from them, and may have the ability to open up to reveal cables, interface ports, or transform into alternative forms, and in some cases can be removed to reveal their "normal" ears, such as with Multi from the To Heart series.
They are predominantly seen on characters who are androids, cyborgs, and mecha musume, or as part of cosplay.