Luckily means by good luck or fortunate chance. When something turns out well not because you planned it but because circumstances happened to work in your favor, that's when you'd say “luckily.”

If you forget your lunch at home but luckily your friend has extra food to share, you benefited from fortunate timing. If you're running late to catch the bus but luckily it's running late too, chance worked in your favor. When a baseball player hits a popup that luckily drops between two fielders for a single, everyone knows skill didn't make that happen.

The word often appears at the beginning of sentences describing how a bad situation was avoided: “Luckily, I double-checked my backpack before leaving.” It acknowledges that things could have gone differently, perhaps badly, but fortune smiled on you instead.

Luckily is closely related to fortunately, though fortunately sounds a bit more formal. Both words recognize that not everything in life comes from hard work or planning. Sometimes you just catch a break, and there's nothing wrong with being grateful when you do.