Complex sentence: In General American, schwa and are the two vowel sounds that can be r-colored (rhotacized); r-colored schwa is used in words with unstressed "er" syllables, such as "dinner".
Split into simpler sentences: In General American, schwa is one of the two vowel sounds that can be rhotacized. This sound is used in words with unstressed "er" syllables, such as "dinner".