UWB is a broad category of radio technologies that transmit simultaneously over a very large range of radio frequencies. While "wide-band" technologies typically transmit over a frequency range spanning around 5 MHz, a UWB transmission may span 500 MHz or more.
Because a UWB signal is spread out over such a wide range of frequencies, a useful signal can be transmitted in whole, while the signal at any one frequency is almost non-existent. In this way UWB can share spectrum with other radio technologies. When properly regulated and implemented, interference is minimal between UWB and other radio systems in overlapping frequency bands.
UWB has unique properties that make it well-suited to precise relative positioning, allowing, for example, your phone to know when it's pointed at another UWB-enabled device, and how far away that device is. This can enable more seamless, intuitive file sharing, and allow your phone to act as a remote key for a car, etc.
UWB for this type of positioning has been included in some high-end phones starting in 2019, and is being standardized as 802.15.4z by the FiRa consortium.
UWB can be used for RADAR applications, allowing a small sensor to sense the motion of nearby objects, such as hand gestures. It can even be precise enough to measure heartbeats.
UWB is also capable of high data rates over short distances. For example, it can be used to send high-resolution video, including VR imagery, to other devices in the same room.
A version of Bluetooth is planned that will use UWB.
UWB the technology should not be confused with Verizon's Ultra Wideband (UW), which is a brand they use to market their mmWave 5G network.