Northern Ireland will continue to follow many of the EU's rules, meaning that lorries can continue to drive across the border without having to be inspected.

However, there will be a new "regulatory" border between Northern Ireland and Great Britain (England, Scotland and Wales). That's because, unlike Northern Ireland, Great Britain won't have to follow EU rules in future.
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From 1 January, some food products arriving in Northern Ireland from England, Scotland or Wales will need to be checked to ensure they meet EU standards - they will need to go through a border control post at sea ports, where paperwork will be checked and some physical inspections will take place.
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Right now the UK and the EU are negotiating a trade deal to try to eliminate new charges - known as tariffs - from being introduced on other goods from 1 January.

However, even if those talks fail, a temporary exemption has been agreed for Northern Ireland. Known as the trusted trader scheme, the vast majority of goods going from Great Britain to Northern Ireland will not face any tariffs.

https://www.bbc.co.uk/news/explainers-53724381

The government is dropping controversial plans to break international law in relation to Brexit, following a behind-the-scenes deal between the UK and the EU, it has announced.

But EU diplomatic sources warned that while the U-turn was a “reset moment” for relations with the UK, it was not a gamechanger for trade talks, which remain deadlocked.

On Tuesday, the government said it would abandon all the Brexit clauses relating to Northern Ireland in the internal market and finance bills following in exchange for promises by the EU to minimise checks and controls due to be imposed on food and medicines going into Northern Ireland from Great Britain from 1 January.

Details on the checks are expected to be disclosed on Wednesday, but the cabinet minister, Michael Gove, and the European commission vice-president, Maroš Šefčovič, announced they had sealed an agreement “specifically for checks on animals, plants and derived products, export declarations, the supply of medicines, the supply of chilled meats, and other food products to supermarkets”.

https://www.theguardian.com/politics/2020/dec/08/brexit-uk-drops-plans-to-break-international-law-as-northern-ireland-deal-is-reached

What was perhaps most interesting about his speech was what it didn’t contain. The biggest breakthroughs that had been reported by journalists were not in Gove’s speech. Although he claimed that ‘Northern Ireland’s supermarket supplies’ would be ‘protected’, Gove gave no details of a trusted trader scheme nor of exemptions or derogations. Instead, there would be ‘a grace period for supermarkets to update their procedures’, and ‘up to 12 months’ for the medicines industry to adapt as necessary to the Protocol rules to maintain those critical supplies. The promise of ‘further measures of financial support, helping businesses and communities to prosper and thrive from the end of the year and beyond’ rings a little hollow when considering the scale of adjustment that NI businesses are having to make in a context of pandemic-induced crisis.

‘Brextensity’ on top of crisis seems as cruel as it is unnecessary. At least now the Protocol should come fully into force without it being the subject of legal action by the EU against the UK. But the significant progress made is just at the level of decision-making, which should have been completed months ago. The lows and highs of the drama at the start of this week may have played well to a certain audience, but they have felt insulting and exhausting to those who simply want to know what to do to keep in business in less than three weeks.

Northern Ireland is not ‘sorted’. There are two things Northern Ireland urgently needs.

First, detailed information and clear answers to questions, and reasonable time and ‘grace’ to adapt. Moreover, the operational systems for the new procedures need to be at least functional. Obviously. And, secondly, above all else, we need a UK-EU deal. It has always been the case that the further apart the UK and the EU sit, the more difficult it would be for Northern Ireland to remain in the middle, closely attached to both. If there is No Deal, then no amount of mitigations or packages (or, indeed, sovereignty-swagger) will compensate for the disruption it will cause to the UK’s internal market and, longer-term, Northern Ireland’s place within it.

https://blogs.lse.ac.uk/brexit/2020/12/10/48-hours-of-brextensity-northern-ireland-is-not-sorted/