Tor: What do Tor Browser’s security levels change?

Short answer

Standard keeps normal website features enabled. Safer restricts some features, including JavaScript on non-HTTPS sites; Safest disables JavaScript by default and limits other active content. Higher levels reduce some attack surfaces but can break interactive websites. Choose the level based on your risk and what must work, and understand that it does not replace careful browsing.

Sources

Tor Project: Security levels.