Complex sentence: Generally, the aim is to associate the attacker's MAC address with the IP address of another host (such as the default gateway), causing any traffic meant for that IP address to be sent to the attacker instead.
Split into simpler sentences: Generally, the aim is to associate the attacker's MAC address with the IP address of another host (such as the default gateway). Any traffic meant for that IP address would be mistakenly sent to the attacker instead.