When they say 'spurious energy modes', read 'hourglassing'.
FEA codes will introduce artificial energy to try and restrain hourglassing modes. The amount of hourglassing in the model is reflected in this value.
A rule of thumb that I have heard is that this energy value should not exceed 1-5% of the value of the strain energy.
For fully-integrated elements, this value is zero since they don't undergo hourglassing (hence no energy is input to resist this deformation).
I hope this helps. Let me know if you have any follow-up questions, or if this isn't clear.
Brad