What does the "rectangle with waves" logo to the right of the trackpad mean on my Dell Precision M4700?

This icon indicates an NFC reader. It is used for two-factor authentication using a contactless smart card and a password.

This means you can log into your computer by both tapping a smart card to the icon and typing your password, offering added security. This is often used in corporate environments where high security is required.

For more information, see page 79 (PDF page 50) of this Dell document. The reader supports several NFC protocols, including iCLASS and MIFARE.

It appears to be the contactless smart card reader. It should appear as "Broadcom Unified Security Hub CV w/fingerprint sensor" in the Device Manager.

It's a Contactless Smart Card reader, according to the manual (see p81). No word on NFC in the manual.