It means that one should never lose hope because nothing is ever completely dark, there is always something good to which we can cling.
Slow: Is alike with foe, because they both rhyme, and is different, because it means to be moving at a low speed. Foe: Alike in the sense that it rhymes, and different because it means your enemy or opponent. These two words can also be alike, because they both mean "annoying things".
That is an example of an aliteration, or giving life to lifeless things throguh description.\
