Only read this is you passed the practice test! Else go back to remedial.
There's three types of assimilation in phonetics. Assimilation needs that something is taken over by another thing, and changes because of that. In the case of phonetics, it's phonemes that are taken over and changed.
It's the difference between how a dictionary says to pronounce things versus how things are pronounced in speech.
Next week you'll spend more time on these, but for now an introduction. For further preparation, read pages 139 - 145 of your book.