Cues are the reminder we need to take action - without them, the desired behaviour won't take place.
You get lots of external cues in your life - such as alarms, or prompts from your external routine. When you wake up - this is a prompt for you to do the next thing in your day.
Emotions can also trigger cues. If you're bored, what app do you reach for? Perhaps Facebook, Instagram, Twitter. When you're lonely, what do you reach for?
This is how cues work and they're crucial to a products success. Without cues, it can be hard for users to betriggered to come back.