Answer

Secure Boot is an option, introduced to most consumers in Windows 8, in the UEFI settings, which some people might call the BIOS. When enabled, your computer will only load drivers or operating system loads that have an acceptable digital signature. This means that Secure Boot ensures harmful software doesn't attempt to load during the boot process.

This may become disabled for a variety of reasons. The most likely is that it was disabled to allow the computer to boot from an external device or to boot a previous version of Windows. In the original version of Windows 8, Windows gave no indication that Secure Boot was disabled. Windows 8.1 changed this by displaying a watermark on the desktop.

If you intended Secure Boot to be disabled, there is no problem. The watermark can be safely ignored, but there is no setting to remove it.

If you did not intent Secure Boot to be disabled, you can enable it. Open the Charms by swiping in from the right, moving the mouse cursor to top-right or bottom-right corner, or pressing Windows+C, then select the following:

Settings > Power > Shift + Restart > Troubleshoot > UEFI Firmware Settings

In the UEFI Firmware Settings, navigate to the Security tab and set Secure Boot to Enabled.

For more information on the watermark, please see this article by Microsoft.

"Secure Boot isn't configured correctly" watermark on the desktop