"Rest mode" refers to a state or setting in which something is temporarily inactive or not in use, allowing it to conserve energy or be ready for future use. Full definition
Make sure your console is set up for automatic downloads while in rest mode and your PS4 will do all the work for you and start downloading as soon as files become available. (techradar.com)
If it's a longer flight, going through a bedtime routine of washing your face and brushing your teeth can help set your body into rest mode. (inc.com)
This will allow the Remote Play application to remotely wake your PS4 from rest mode so you can quickly connect and play games. (howtogeek.com)