I've been trying to get my head around wing design and have been following a few YouTube videos. When people talk about aspect ratio (AR) on a wing and give a value (Piper PA28 having an AR of 5.7) what is the 5.7? Is it wingspan length? Square metre of coverage? Or something else? I'm confused.
The Aspect Ratio ($AR$) is defined as the ratio (hence the name) between the wingspan squared $b^2$ and the area of the wing $S$.
Since both $b^2$ and $S$ are measured with a length squared (e.g., $m^2$), $AR$ is an dimensionless quantity.
To answer the follow-on question of TomMcW in the comments, $b$ is the distance between the wingtips, no matter the shape of the wing.