The adjective, רַבָּֽה׃ informs the verb for being moved and the adverb, "not," creating a sense that while we might be shaken, we shall not fall completely over.

We may lose our balance a bit, but we regain it. Our rock and salvation are strong even when we are not.

"For God alone my soul waits in silence;
from him comes my salvation.
He alone is my rock and my salvation,
my fortress; I shall not be greatly shaken."