Doing something joyfully means that you're brimming with happiness as you do it. If you're in a bad mood, watching your little sister and her friends run around the playground joyfully might cheer you up.
If you love snow, you'll joyfully welcome those first white flakes, and if singing with your friends makes you deliriously happy, you'll joyfully join in when they begin to croon. Your dog might like to frolic in the park joyfully, and your parents might applaud joyfully when you receive your high school diploma. Joyfully comes from joy and its root, which means "I rejoice."