A more thorough explanation:

A civil union is a legal relationship between two people that provides legal protection to the couple at the state level. It is similar to marriage, but was created primarily as a means to provide recognition in law for same-sex couples.

For example, if a same-sex couple is in a civil union, they get inheritance rights, employment benefits, property, parental rights, and other legal protections.

The first state in the U.S. to recognize civil unions was Vermont. The state supreme court ruled in Baker v. State of Vermont that the state had to extend to same-sex couples the common benefits and protections granted to those who marry under Vermont law. The Vermont legislature passed legislation creating the opportunity for same-sex couples to obtain a “civil union” license.

However, the United States Supreme Court ruled in Obergefell v. Hodges that same-sex marriage is legal in every state and each state must recognize a same-sex marriage performed in other states. In light of this U.S. Supreme Court decision, civil unions may now become less common.