In grammar, an appositive word or phrase is one that describes the word or phrase right next to it. "My cat" is the appositive phrase in the sentence, "I'd like you to meet Burrito, my cat."
While this adjective, strictly speaking, means "beside" or "adjacent," it's most commonly used to talk about grammar. If you say, "I saw Lady Gaga, a famous singer, at the supermarket," you've used the appositive phrase "a famous singer," which describes Lady Gaga. Appositive can also be used as a noun. In the song "Do-Re-Me," from The Sound of Music, the appositive "a deer" adds information, explaining exactly what a "doe" is.