9 Awake, awake, put on strength, O arm of the Lord; awake, as in the ancient days, in the generations of old. Art Thou not It that hath cut Rahab, and wounded the dragon?
10 Art Thou not It which hath dried the sea, the waters of the great deep; that hath made the depths of the sea a way for the ransomed to pass over?
Here, the people were calling upon the Lord to awake and put on strength as in ancient days when He supernaturally intervened at the time Israel was in danger of extermination. They were so desperate that they made their request in the form of a command to God, as He had told them to do (45:11). Then, they asked questions concerning His identity with the miracles of the past in leading Israel out of Egypt, and concluded that since He was Deliverer in those days He would rescue them again, bring them back to Zion, and restore them as an eternal nation under their Messiah.
The arm of the Lord is figurative of His power, as in many scriptures. You may find it a blessing to do a word search on that phrase.
Rahab is the poetic name of Egypt (Ps.87:4; 89:10).
The dragon in v. 9 is a crocodile, which infested the waters of the Nile in Egypt.