The term means that ip provides no error checking or tracking. ip assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees

 "Best-Effort"

Explanation: The IP protocol is neither responsible for delivering packets to destination nor those packets arrive in the same order than they were transmitted, i.e., does the "Best Effort", that's all.

These functions are responsibility of the transport layer, like TCP in the TCP/IP Suite.