Generally considered one of the "layers of light," accent lighting is directional light aimed at an object or feature in a space. It is a way to make something stand out in the space, to create a visual separation.
Flood lights, spot lights, track lights and even wall sconces do a good job at this. Shining a light at a painting or framing a fireplace mantle with two sconces can draw the eye to the location in the room that you want to highlight.