What noun is the appositive phrase describing in Sherrie was very upset when movers dropped her stereo system a gift from parents?

The appositive phrase "a gift from parents" is describing the noun stereo system.

Here's how to break it down:

* Sherrie was very upset... This tells us the subject is Sherrie.

* ...when movers dropped her stereo system... This tells us what caused Sherrie's upset.

* ...a gift from parents. This phrase describes the stereo system, clarifying its origin.