Do not be anxious about anything. Instead, in every situation, through prayer and petition with thanksgiving, tell your requests to God. And the peace of God that surpasses all understanding will guard your hearts and minds in Christ Jesus.

Paul wrote this letter while under guard in Rome, waiting on a legal outcome that could have gone as far as execution. That is worth holding onto, because the instruction not to be anxious lands very differently coming from someone comfortable than from someone in a cell.

The structure of what he says is specific. He does not say stop worrying. He says take the thing you are worried about and say it out loud to God, by name, in detail, and to do that with thanks rather than only complaint. It is a transfer, not a suppression.

The word translated guard is a military term. It is what a garrison does around a city. Paul is not describing peace as a mood that settles over you when circumstances improve. He is describing it as something posted around your thinking, doing a job, whether or not the situation has changed.

He also says this peace surpasses understanding, which is a stranger claim than it first sounds. He means you will not be able to account for it. It will not be proportional to how the situation actually looks.