Albert Barnes Commentary Psalms 114:4

"The mountains skipped like rams, The little hills like lambs." — Psalms 114:4 (ASV)

The mountains skipped like rams - As flocks in their playful leaps. They seemed to move from place to place; everything seemed to be unsettled, and acknowledged the presence of the Omnipotent One. The word rendered “skipped” means to leap for joy; to dance. See the notes at (Psalms 29:6). The reference here is to the agitations and commotions of the peaks of Sinai, when God came down to deliver the law (Exodus 19:16–18).

And the little hills like lambs - Hebrew, Like the sons of the flock. The reference here is to the less prominent eminences of Sinai. The lofty hills, and the smaller hills surrounding, seemed to be all in a state of commotion.