Declarative is ordering from a menu; imperative is cooking the meal yourself.

In a declarative style you describe the end result for the current data, and the library works out the steps. In the imperative style you write the steps yourself: find this node, set its text, toggle that class. React's pitch is that you stop managing those steps, so one forgotten update cannot leave the page half-changed.

Read it once, then a question tests whether the trick stuck.