From Mamya's link:
//The CTC means goods do not need to complete import and export declarations each time they cross a new border and traders only have to make customs declarations and pay import duties when they arrive at their final destination//
Also here, from UK Trade Forum:
//The Common Transit Convention allows a suspension of customs checks and payment of duties until the goods reach the destination country.
...
Signing the Convention could potentially address some issues for some shipments. Under transit, it means goods from outside the EU could pass through the UK as a third party with duties suspended to another destination – Ireland for instance. Likewise, for Ireland the UK is a land bridge for its exports to other countries – it’s estimated in a report for the Irish Government that 53 per cent of Irish goods exports (measured in volume) to other countries are transported via the UK. Transit would be vital to maintain this trade.
...
But there are still many issues the current provisions wouldn’t solve. For a start, any good going through the normal import process would still create huge issues for ports like Dover.
In addition the current facilities and the number of approved traders are not prepared to handle the scale of trade requiring clearance post-Brexit. These facilities would need significant infrastructure investments and capacity upscaling, which take time and money. ... //
https://uktradeforum.net/2018/04/24/securing-european-transit-for-uk-trade-post-brexit/
I.e., if we are the destination country, then customs checks and duties would surely still apply when goods arrive here.