A Content Security Policy (CSP) is a security mechanism that addresses the weakness of same-origin policies by approving specific content types, allowing trusted entities to have access rights while preventing third-party entities from doing so. The CSP has two modes: enforcement and report-only mode, with report-only mode collecting errors without actively blocking resource loading or execution. To update a CSP, it's essential to refine and improve rules, as seen in the example where outdated libraries caused issues due to eval() expressions not allowed in CSP rules. The newest iteration of CSP, version 3, is about to be fully released, bringing simplified configuration and improved resilience to cross-site scripting attacks, and it's crucial to define directives in the order of their levels for optimal parsing by browsers.