In ordinary speech
话头 is just 'thread/topic of talk'. In Chan (Zen) Buddhism, however,
话头 is a technical term: a critical short phrase — most famously 'who is dragging this corpse around?' — that the meditator holds in mind to break through discursive thinking. The technique was systematized by Song-dynasty Linji master
大慧宗杲 (Dahui Zonggao) and remains central in modern Chan retreats.