Out-of-order packets are received in a different order to which they were sent. This can be because of asymmetric routing, load balancing, route changes during transmission or other variables. TCP includes sequence numbers in packets so the receiver can put the packets back in to order. Even with TCP sequencing packets, out-of-order packets can cause issues performance or reliability issues on the network.
To reorganize the packets the CPU will buffer them, causing a hit on performance. If enough happen the receiver will send duplicate ACKs, causing the fast retransmission to occur. This results in reduction of the TCP window size and less throughput.
When too many packets arrive out of order, TCP will send retransmission packets, similar to if you have packet loss.