A more thorough explanation:

Marital privilege, also known as spousal privilege, is a legal concept that protects private spousal communications and prevents any testimony against a spouse from being used in court. There are two types of marital privileges: marital communications privilege and spousal testimonial privilege.

In both civil and criminal cases, communications made between spouses during the marriage are privileged if the communication is intended to be private and made in reliance on the sanctity of marriage. This privilege can be asserted even if the marriage is terminated because of divorce or the death of one spouse. In most jurisdictions, both spouses hold the privilege, and any party can assert it by refusing to testify about spousal privileged communications or by preventing the other party from doing so. However, some states rule that only communicating spouses can assert the privilege. The burden of proof is on the opposing party, who should prove that particular words and acts are not intended as private communication.

For example, if a husband and wife have a private conversation about their finances during their marriage, neither of them can be compelled to testify about that conversation in court if one of them is called as a witness.

In criminal cases, the spouse of a criminal defendant who is called as a witness by the prosecution may choose to testify but cannot be compelled to testify against his or her spouse about events that occurred before and during the marriage. While the privilege can be waived by the witness spouse, even if the defendant spouse objects. In a minority of jurisdictions, the defendant spouse holds the privilege to prevent the witness spouse from testifying, regardless of the witness spouse’s willingness.

For example, if a wife is called as a witness in a criminal trial against her husband, she cannot be forced to testify about any events that occurred during their marriage.

Marital privilege does not apply if the private communication is revealed to third parties, one spouse is suing the other, or when one spouse is charged with a crime against the other or their children. A judge must decide whether to recognize an exception to the marital privilege by giving judicial notice of legislative facts, which are policy facts related to legal reasoning and the lawmaking process.

For example, if a husband is charged with domestic violence against his wife, she can be compelled to testify against him in court.