An a-line is a type of clothing that is shaped like an "A", kind of like the letter you see in the alphabet.
Imagine taking a piece of paper and folding it in half. If you cut the paper diagonally from the top fold to the bottom corner, you would have the shape of an a-line skirt.
A-line clothing is designed to be wider at the bottom and narrower at the top. This means that it will fit snugly around your waist and hips, but then it will flare out at the bottom.
A-line clothing can include dresses, skirts, and even jackets. It's a popular style because it is both comfortable and flattering on many different body types.
So if you ever see a dress or skirt that looks like it's shaped like the letter "A", you can now be a pro and say, "oh, that's an a-line!"