This is the period of daytime shortly after sunrise or before sunset, during which daylight is warmer and softer than when the Sun is higher in the sky. There are no sharp shadows during this magic hour because the sun has just set (or hasn't risen yet). The light you will get is magical and gives photos a more natural and colourful look.
Because this period is relatively short (between 20 to 30 minutes), it means that we, as outdoor photographers, need to be on location quite early. Waking up at 4 AM is not exceptional, even on vacation!