Appositive is defined as a noun, a noun phrase, or a noun clause which sits next to another noun to rename it or to describe it in another way. The sentence can still be completed without an appositive but adding it can present more information about the subject.
It means that one should never lose hope because nothing is ever completely dark, there is always something good to which we can cling.
Past is done, since you have done something. Present is doing, since you are doing something.