What does Isaiah 12:5 mean?

Thanking God for His power and mercy and salvation for us is both personal and public. It is both an expression from each of us to Him and a declaration to all who hear us that He is great. That is what God intends for us. We are not to hide our joy in our salvation.

This chapter has described how the people of the Lord will respond to His victory over the earth and the comfort He gives to them when He establishes His kingdom. They will express heartfelt thanks and acknowledge that He has always been their salvation, strength, and song (Isaiah 12:2).

The author brings further attention the fact that the Lord's people will publicly proclaim the Lord's greatness to the world. They will sing praises to Him in the full view of all who listen. They will tell the stories that reveal His glory. They will be intentional about broadcasting the power and goodness and righteousness of the Messiah to every corner of the globe.