A more thorough explanation:

An interested witness is someone who has a personal interest in the outcome of a legal case. This means that they will be affected by the decision that is made in the case. For example, a defendant in a criminal trial is an interested witness because they have a personal interest in the outcome of the trial.

In some cases, the judge may tell the jury to consider the interest that a witness has in the outcome of the case when deciding whether to believe their testimony. This is because a witness who has a personal interest in the case may be more likely to lie or exaggerate in order to get the outcome they want.

For example, if a witness is testifying in a case where they are suing someone for money, the jury may be more skeptical of their testimony because they have a financial interest in the outcome of the case.