No this is not a title for a scary movie! Let's go through what is the title all about...
Thrillers are tall, upright plants that add the height to the arrangement. Thrillers are the great way to add a vertical aspect to your containers. Many thrillers are architectural: plants with structural, eye-catching form that can serve as a sturdy backbone or rugged framework for a scrim of less substantial plants.
Fillers are covering plants that spread to fill the gaps between the Thrillers and spillers, more finely textured plants that surround and weave through the thriller. They fill up the pot while embracing the thriller. Often, they help by hiding the bare knees.
Spiller plants are those that spill over the edges of the container and cascade down. The main role of a spiller, however, is to sprawl over the side of the container, softening its edges and tumbling toward the ground. When parts of a container planting touch the earth, the pot looks rooted to its place.
Planting the right combination of thrillers, fillers, and spillers creates a lush, intriguing composition rich in color, texture, and form. It fills out a pot by exploiting space in every available direction—up, down, and sideways.