Your “rock” is your refuge, a high place with a strong foundation that is indestructible and will be with you forever. This is a common expression in today’s English, and was a common idiomatic expression in the Bible in reference to the Creator. (For example, see Psalm 18 verse 2, for example, where two different Hebrew words for “rock” are used in the same verse.)

It's a figure of speech meaning that she is a strong, supportive person, and that he relies very heavily on her support.