A more thorough explanation:

A competent witness is someone who is mentally capable of understanding and remembering what they have seen or heard, and can explain it in court. They must also understand the importance of telling the truth under oath.

For example, a child who is too young to understand the meaning of an oath is not a competent witness. Similarly, someone who is mentally ill or has a personal interest in the case may also be considered incompetent to testify.

In the context of wills, a competent witness is someone who has signed their name to the will and can legally testify to its contents in court.

It is generally assumed that a witness is competent unless there is evidence to suggest otherwise. If a party wishes to challenge the competency of a witness, they must ask the court to exclude their testimony.

For instance, if a witness is called to testify in a criminal trial, the defense may argue that the witness is not competent because they have a personal interest in the outcome of the case. The judge would then have to decide whether or not to allow the witness to testify.