In communication, when a system receives data, it can ACKnowledge that it has received the data by sending something back to the sender. That way the sender knows that the message was received and can e.g. send the next data to the other system. If it does not receive an ACK within a certain time, the sender will know that something is wrong and might e.g. retry.
So if you send something from the PC to the Arduino, the Arduino can reply with something like "OK" or "ACK"; there is also a dedicated ACK character in the ASCII table but Serial monitor will not print it.