Tangling arises when one module implements (part of) several requirements; scattering arises where the implementation of a single requirement is spread across several modules.
Say several requirements in a system have a shared component that implements the issuing of a confirmation to a user that some action has occurred. This is tangled system. If one of these changes so that some change to the confirmation is required, all of the other requirements that issue confirmations are affected by this.
If the implementation of each requirement which issues a confirmation is scattered across several modules, then all of these have to be checked that the change will not affect their operation.
