thinly sliced salmon head cartilage
A food item made from the cartilage of salmon (or sometimes whale) head, sliced thin and often served as sashimi or in salads. The term is primarily used in culinary contexts.
Hizu is thinly sliced cartilage from the head of a salmon.
At this restaurant, vinegared hizu is popular.
The kanji 氷 (ice) and 頭 (head) likely refer to the translucent, ice-like appearance of the cartilage when sliced thin. The reading ひず is a kun'yomi compound reading.