A sound cue is an audio object that encapsulates complex sound design tasks in a sound graph.
You can use a sound cue to modify an existing audio file by using audio nodes on the graph. Some of the things you can do with a sound cue include creating audio loops, fading sounds based on player distance from the source, and creating an audio sequence by using multiple sounds in a single cue.
UEFN provides a number of sound cues, but you can also create your own.