Business term referring to how well a product is selling or the level of demand. Often used in contexts like 売れ足がいい (selling well) or 売れ足が悪い (selling poorly).
Sales of the new product are strong.
Demand for heating appliances increases at this time of year.
Compound of 売れ (continuative form of 売れる, 'to sell') and 足 ('foot, pace'), suggesting the pace or movement of sales.