What does Isaiah 1:20 mean?

The Lord, through the pen of Isaiah, is offering His people an opportunity to repent. To stop their sinful choices and embrace new, good actions of taking care of the least among them. (Isaiah 1:16–17) He has said that if they are willing to obey Him, they will have all they need and more.(Isaiah 1:19) If they will do what is right before the Lord in their hearts and with their actions they will thrive and be protected from their enemies.

Now the Lord adds that if they refuse this offer to repent and continue to rebel against Him, they will be "eaten by the sword". It's a powerful use of poetic wordplay. Either God's faithful people will eat well from the land or they will become the food of their enemies' swords and be destroyed.

The Lord is abridging His covenant with His people (Deuteronomy 30:15–20) to one choice: obey and prosper or rebel and be destroyed. It couldn't be simpler.

He ends this section with a phrase, "the mouth of the Lord has spoken", which means these words are final and just.