What does donor level mean?

I saw a sentence in the documentation http://what-when-how.com/electronic-properties-of-materials/semiconductors-electrical-properties-of-materials-part-2/ that is not well understood.

The distance between the donor level and the conduction band represents the energy that is needed to transfer the extra electrons into the conduction band. (The same is true for the acceptor level and valence band.) It has to be emphasized, however, that the introduction of these impurity levels does not mean that mobile electrons or holes are found in the forbidden band of, say, silicon. The impurity states are only used as a convenient means to remind the reader of the presence of extra electrons or holes in the crystal.

Are the donor level and acceptor level imaginary lines? If so, is the real energy level of the extra electron in the valence band or the conduction band?