CSR BlueCore is a series of Bluetooth chipsets developed by CSR (Cambridge Silicon Radio), which are used in various Bluetooth devices, including Bluetooth dongles, audio adapters, and more. When you see references to CSR BlueCore in DFU mode, it typically means that the device (such as a Bluetooth adapter or module) is in Device Firmware Update (DFU) mode. This mode allows the firmware of the device to be updated.