People of Bible times wore inner tunics and outer robes that covered the entire body (read 1 Samuel 19:24 and 1 Kings 18:46). The bare arm could be exposed quickly by pulling it from under this loose clothing. “Baring the arm” was a metaphor for swift, decisive action.
When God delivered His people, according to Isaiah in this verse, He would do so by baring His arm-powerfully and forcefully.
ISAIAH 52:10 – The LORD hath made bare his holy arm in the eyes [sight, NIV] of the all nations.