TCP is a reliable connection. That means it makes sure that the data sent by the sender is completely received by the receiver. To starts a communication between the sender and receiver, TCP follows a three-way handshake. SYN denotes the synchronization packet and ACK stands for acknowledgement:

The sender starts by sending a SYN packet and the receiver replies with SYN-ACK. The sender sends back an ACK packet followed by the data. In SYN flooding, the sender is the attacker and the receiver is the victim. The attacker sends a SYN packet and the server responds with SYN-ACK. But the attacker does not reply with an ACK packet. The server expects an ACK packet from the attacker and waits for some time. The attacker send a lot of SYN packets and the server waits for the final ACK until timeout. Hence, the server exhausts its resources waiting for ACK. This kind of attack is called SYN flooding.