Free trade happens when people can buy goods and services from whomever and wherever they like, without obstruction from governments. This allows people to specialise in whatever they do best and maximises consumer welfare.
Arguments against tariff-free trade usually boil down to saying that British families must pay higher prices to subsidise farmers or steelworkers who cannot compete with more efficient producers elsewhere. Worries about ‘unfair’ competition, or the benefits of seif-sufficiency, rarely stand up to serious examination.