A TCP RST (reset) attack is meant to abnormally terminate legitimate TCP connections by sending a random packet with the RST bit set.
In the packet stream of a TCP connection, each packet contains a TCP header and every header contains an RST bit. If this bit is set to 1, it instructs the receiving computer to immediately terminate the TCP connection. Following this instruction, the sending computer does not forward any more packets through the connection's ports, and discards any further packets it receives with headers indicating they should be sent to that connection.
A TCP reset terminates a TCP connection instantly.
Refer to Configuring use case 2 - protection against TCP RST attacks.