A more thorough explanation:

QUOD ERAT FACIENDUM

Quod erat faciendum is a Latin phrase that means "which was to be done." It is often abbreviated as Q.E.F. and is used to indicate that a proof or demonstration has been completed.

For example, if a mathematician is trying to prove a theorem, they may end their proof with "quod erat faciendum" to indicate that they have successfully demonstrated what they set out to prove.

Another example could be in a scientific experiment where the researcher has successfully completed all the necessary steps to reach their conclusion. They may end their report with "quod erat faciendum" to indicate that they have achieved their goal.

The examples illustrate how quod erat faciendum is used to indicate the successful completion of a task or goal. It is often used in academic or scientific contexts to indicate that a proof or experiment has been successfully completed. The phrase is a way of saying "that's what needed to be done" or "mission accomplished."