The LEGO NINJAGO Movie Video Game

Side quests are unlocked as you play through the story. There are 25 of these located throughout Ninjago City and are represented by a white compass icon on the mini-map. Make your way to the icon to talk to the quest giver. The quests involve you going around various areas fetching items, defeating enemies or solving puzzles, to name a few. Each quest only takes a few minutes to complete.

For each quest completed you earn a gold brick. Complete all 25 to unlock this achievement.

Thanks to packattack04082 for the video: