Apposition is when two nouns or phrases are placed back to back and both nouns refer to the same thing. For example, in the following sentence,
We visited the home of Harriet Beecher Stowe, the author of Uncle Tom’s Cabin.
“Harriet Beecher Stowe” refers to the same person as “the author of Uncle Tom’s Cabin”. Hence, we would say that “the author of Uncle Tom’s Cabin” is in apposition to Harriet Beecher Stowe.