A browser restart is the moment when a downloaded browser update is loaded into memory and the new, fixed code actually starts running. Until that happens, the browser may still be using the older vulnerable version, even if the patch has already been installed on disk. In security terms, the restart is what turns an update from a promise into real protection.
This matters because browsers are frequent targets for exploit chains, drive-by attacks, and malicious web content. If users leave the browser open for days, risk can persist long after a patch is released. Defenders reduce that gap by enforcing restart prompts, tracking active versions, and checking for stale sessions on managed devices. Attackers often benefit from delayed restarts, since unpatched instances remain exposed while the organization thinks it has already updated.