Error correction mode is an optional transmission mode built into Class 1/2/2.0 fax machines or fax modem

Fax modem

A fax modem enables a computer to transmit and receive documents as faxes. A fax modem is like a data modem but is designed to transmit and receive documents to and from a fax machine or another fax modem. Some, but not all, fax modems do double duty as data modems. As with other modems, fax modems...

s. ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line

Telephone line

A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

 noise

Noise

In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...

.
The page data is divided into what is known as Octets (small blocks of data

Block (telecommunications)

In telecommunications a block is one of:* A group of bits or digits that is transmitted as a unit and that may be encoded for error-control purposes....

). Once the receiver

Receiver (Information Theory)

The receiver in information theory is the receiving end of a communication channel. It receives decoded messages/information from the sender, who first encoded them. Sometimes the receiver is modeled so as to include the decoder. Real-world receivers like radio receivers or telephones can not be...

 has received all the Octets it examines them (using check-sums) and then advises the transmitting fax of any Octets that are in error. The transmitter then need only resend the blocks in error rather than the whole page. This generally means an ECM coded fax will be more likely to succeed on a noisy line.
ECM is the norm rather than the exception. Some fax machines have the capability to enable or disable this function.