What does the absolute value sign mean when used around the differential part of an integral?

Question

The Wikipedia entry on Radon transform shows an equation like the following: $$ \int_L f(\mathbf{x})|\mathbf{dx}|. $$ What does absolute value around $$|\mathbf{dx}|$$ mean in an integral?

I understand that for the Radon transform we want to integrate the function along the line $L$, but I don't understand the notation in the equation I listed above.