It an imperative command telling to copy/import another image, not different from importing a library in any other imperative language

It is the most declarative thing in a Dockerfile. It becomes an imperative solution very quickly.

Here's an example Dockerfile:

FROM McDonalds Cheeseburger HappyMeal

RUN Get ketchup packets

RUN Squirt ketchup on fries

RUN Dump happymeal on plate

The first is declarative, a menu, you point at what you want and software makes it so.

The rest is a recipe, an imperative, a command, you tell software to perform, and software makes it so.