The meaning is exactly what it says.
To make a long story short:
The underlying problem is that the integrations are performed in the element's parent domain(i.e. for bilinear quads the parent domain is a square with center at (0,0) and side length=2). A mapping (transformation) is used between this parent domain and the spatial domain occupied by the element in the FE model. The Jacobian of this transformation is used for performing the numerical integrations. However if the element gets to distorted the Jacobian starts going to zero, causing numerical problems (also if the Jacobian were zero the mapping would not be invertible). In order to preclude the Jacobian to go to zero the software checks the spatial element geometry, for example, the element's angles.