What does it mean that User Datagram Protocol (UDP) is unreliable?

What will be an ideal response?

Unlike TCP, User Datagram Protocol (UDP) is a connectionless, unreliable delivery service. Connectionless means that the sender does not have to first establish a link to the receiver before beginning to transmit data. Unreliable does not mean worse or unacceptable. With UDP, unreliable simply means that the sender does not guarantee to the receiver that all of the transmitted data packets will arrive. Also, if a message requires multiple packets, the packets may or may not take the same route to the receiver, and thus may arrive out of sequence. It is up to the receiver to verify that the data expected has in fact been received and to put the packets back in order, if needed.