To spoof an address is to use a fake source address (IP or MAC address) when sending data.
Many kinds of attacks involve spoofing; it’s not necessarily an attack on its own itself. However, there are attacks whose primary tool is address spoofing. For example, a DHCP exhaustion attack in which an attacker uses spoofed MAC addresses to flood DHCP Discover messages. The target server’s DHCP pool becomes full resulting in a denial-of-service to other devices.