Standard Definition:
A network interface card (NIC) is a hardware component, typically a circuit board or chip, which is installed on a computer so it can connect to a network. Modern NICs provide functionality to computers, such as support for I/O interrupt, direct memory access (DMA) interfaces, data transmission, network traffic engineering and partitioning. [Source]
Simplified Analogy:
The NIC card gives the computer the ability to be connected to a network. Each NIC has a unique number that identifies it known as the MAC (Media Access Control) address. This can be likened to a chip-embedded ID card that allows a staff access to organization’s facilities.
The computer on which the NIC is installed can uniquely be identified with this MAC address. In cases where device identity is crucial e.g., did this specific laptop access confidential files? The MAC address can serve as a piece of evidence to confirm or refute such claims. Although MAC addresses are expected to be globally unique in principle it is possible for a user or software to manually change a device’s MAC address to any desired value, effectively spoofing or cloning another device’s MAC address. This can be done for legitimate purposes, such as bypassing MAC address filters, or for malicious purposes, like impersonating another device.