Lifetime goals are on-going goals that escalate in difficulty as you complete them, and are accessed in the Goals menu. Some reward consistent behavior, such as reaching a certain level. Others reward supreme puzzle skill, such as solving a puzzle in as little as two turns!
You’ll almost certainly complete a number of lifetime goals as you tour the world and play against friends, so make sure to check the challenges tab periodically! You don’t want to miss out on any rewards you earn.