If one is going to err, one should err on the side of liberty and freedom.

The line frames a moral and political rule-of-thumb for decision-making under uncertainty: when outcomes are unclear and mistakes are possible, it is better to risk granting too much freedom than to risk unnecessary restriction. Implicit is a view that liberty has a high default value and that curtailments of freedom carry especially grave, often irreversible costs—abuse of power, repression, and the erosion of rights. At the same time, the phrasing acknowledges that governance and security dilemmas can force imperfect choices (“err”); Annan’s emphasis is that the burden of justification should fall on limiting liberty, not on exercising it.