A cumulative sentence (also sometimes called a loose sentence) is an independent clause followed by one or more modifiers. Essentially, you use words, phrases, and clauses to expand on or refine the main idea of the sentence. The effect can be natural as it parallels what often happens in speech. Here's a cumulative sentence from “Georgia O'Keeffe," an essay by Joan Didion:
She is simply hard, a straight shooter, a woman clean of received wisdom and open to what she sees.
The first part of the sentence is an independent clause. It can stand on its own: “She is simply hard." The rest of the sentence modifies that independent clause, helping to more precisely define what “simply hard" means in this instance. Notice there are a series of modifiers: a straight shooter, a woman clean of received wisdom, a woman open to what she sees.
This next sentence from the short story “Listen Carefully as Our Options Have Changed" by Andre Dubus III, has less expansion, but is still cumulative:
At first there is only the coffee table in front of him, a swatch of sunlight across its glass surface.
The image of the sunlight enriches the description of the coffee table.