JTAG_SHIELD	0x1 = JTAG debugger is disabled
This means that since the end user has access to JTAG at system reset and during ROM code execution before control is transferred to user software, a JTAG user can modify the boot flow and subsequently disclose all CPU information, including data-encryption keys.
(informative)
The default value of this register bit should be set to 1 to prevent the JTAG from being enabled at system reset.