A renderer sandbox is the restricted browser environment where web pages are executed after the browser has separated them from more privileged system functions. Its job is to limit what compromised content can do: read files, launch programs, access devices, or interfere with the rest of the browser should be heavily constrained.
This matters because modern browser attacks often start with a memory corruption bug or scriptable flaw in the renderer. If an attacker can only crash a tab, the sandbox is doing its job; if they can chain another flaw to escape the renderer, they may reach a stronger browser or operating-system context. Defenders rely on layered controls such as sandboxing, Site Isolation, patching, and endpoint monitoring to reduce that blast radius and catch suspicious child processes or post-browser activity.