Four important observations spring from this verse. The first is an appeal to God's grace. Isaiah made this request even though he had spoken words of judgment knowing that brutal conquest would decimate God's people in the very near future. He cried out for grace to pour out from God. He hoped in the power of God to fulfill His purpose in the restoration of all things. All who really desire God will cry out for grace. It is the heart's calling when it knows God.
Seasoned with this appeal is an awareness that realistically the people would necessarily have to wait for just the right time and place to know God's restorative gracious action. They would wait on The Lord. Much of the strengthening that God does in us is on His eternal time scale. And we wait... and we wait... and wait... patiently and sometimes interrupted by fits of our own selfishness. But in order to see the end of the matter that God has for us, we MUST wait.
The third request is poetic. Isaiah asks God to be Israel's "morning arm". We work with our arms. We provide for our lives with that work. This is an appeal to trust in God's strength and provision. And Isaiah wants that attitude to start every new morning, implying that only God's strength will get His people through each day. And God must be "our arm every morning" if we are to make every day truly count for eternity.
The final request is for God to be "our salvation in the time of trouble". Really, God is always our salvation. But in trouble we are tempted to take our eyes off of Him until we are overwhelmed by our great need and inability to save ourselves. We need God to save us from our massive failure to control our own sinful impulses. He is our only Savior.