What is the SI unit of force?

The newton, defined as the force that accelerates one kilogram by one metre per second squared. It is roughly the weight of a small apple held in the hand, which is a coincidence nobody planned and everybody enjoys.

The distinction it enforces is between mass and weight, which ordinary language runs together. Mass is how much matter there is and does not change; weight is a force and depends on where you are. A 70-kilogram person weighs about 686 newtons on Earth and about 114 on the Moon, while their mass is 70 kilograms in both places - and in orbit they are weightless with their mass entirely unchanged.

Kilograms are used as a weight unit everywhere in daily life and this almost never matters, but it matters at the edges. Bathroom scales measure force and display mass, assuming Earth's gravity - and gravity varies enough across the planet that the same scales read slightly differently in Oslo and Singapore. The kilogram itself was defined by a physical cylinder in Paris until 2019, when it was redefined in terms of the Planck constant, so that the last unit tied to an object became tied to a number instead.