Rendered beef fat. Chongqing hot pot bases are traditionally fried in it with chilies and Sichuan pepper. In Hong Kong the word means butter, which the mainland calls 黄油.
In mainland China 牛油 mostly means beef tallow, the fat behind Chongqing hotpot, and butter is usually 黄油. 牛油 for butter is Cantonese and Taiwan usage, also common on bakery labels.