These are both legal terms. Marriage is a legal contract. Many states of the U.S. have "community property" laws: a spouse becomes half-owner in everything the other owns. If one spouse is much wealthier than the other, they may not want that. They may only agree to become married on condition that they keep their wealth separate. This can be accomplished by a prenuptial agreement. Hollywood, once the home of the U.S. movie industry, is in California, a community property state. Thus, prenuptial agreements are common between Hollywood actors. In the U.S., everyone is advised to write a "will," and most people do. The will lets you say who gets your property after you die. The people who get the property are the beneficiaries. A "sole beneficiary" means that the will has only one beneficiary, either because only one was named, or because only is alive at the time when the will takes effect.

To add to James, "beneficiary" is a legal term. When you write a will, you specify who will receive your property when you die i.e. your beneficiaries.

A prenuptial agreement defines how the property will be split if a married couple divorces. The sole beneficiary is the only person who receives the benefits of a legal document.