What is $ds$ in centroid equation's $\int y\space ds$?

Question

I'm looking at the equations for finding the centroid.

For the y-coordinate it is: $$\frac{\int y\space ds}{\int ds}$$

${\int ds}$ is defined to be the curve length. But what then is $ds$ in $\int y \space ds$?
Is it the derivative of ${\int ds}$?