A more thorough explanation:

The missing-evidence rule is a legal principle that states that if a party fails to present evidence at trial that they had control over and that would have been relevant to the case, the jury can assume that the evidence would have been unfavorable to that party.

For example, if a plaintiff in a personal injury case fails to present medical records that they had access to, the jury may assume that the records would have shown that the plaintiff's injuries were not as severe as they claimed. Similarly, if a defendant in a criminal case fails to present an alibi witness that they had the ability to call, the jury may assume that the witness would have testified against the defendant.

The missing-evidence rule is based on the idea that parties have a duty to present all relevant evidence to the court, and that failure to do so can be seen as an admission that the evidence would not have helped their case.