The transition period is meant to give both sides some breathing space while a new free trade agreement is negotiated. This is needed because the UK will leave the single market and customs union at the end of the transition. A free trade agreement will allow goods to move around the EU without checks or extra charges. If a new one cannot be agreed in time, then the UK faces the prospect of having to trade with no deal in place. That would mean tariffs (taxes) on UK goods travelling to the EU and other trade barriers. Aside from trade, many other aspects of the future UK-EU relationship will also need to be decided.