When a NIC is placed in promiscuous mode, it will pass all data it receives onto the CPU. Normally, a NIC will ignore data not intended for it's IP or MAC Addresses.
Wikipedia
In computer networking, promiscuous mode is a mode for a wired network interface controller (NIC) or wireless network interface controller (WNIC) that causes the controller to pass all traffic it receives to the central processing unit (CPU) rather than passing only the frames that the controller is specifically programmed to receive. This mode is normally used for packet sniffing that takes place on a router or on a computer connected to a wired network …