The watt is a unit of power, i.e. of energy/time.

1 watt = 1 joule/1 second

The watt-hour is a unit of energy defined as the energy delivered by a power source of 1 watt during 1 hour.

1 hour = 60 minutes = 3600 seconds

Therefore 1 watt-hour = 1 joule/second x 3600 seconds = 3600 joules.