A more thorough explanation:

Same-sex marriage is when two people of the same sex or gender identity get married. For example, two men or two women can get married to each other.

In 2003, Massachusetts became the first state in the United States to legalize same-sex marriage. After that, many other states also started to consider legalizing same-sex marriage. Over the next decade, several other states legalized same-sex marriage. In 2015, the U.S. Supreme Court made a decision in the case of Obergefell v. Hodges that legalized same-sex marriage nationwide. This decision included the remaining 14 states that had not previously allowed same-sex couples to marry. The court made this decision based on the Due Process and Equal Protection clauses of the Fourteenth Amendment of the U.S. Constitution.

For example, if two women love each other and want to get married, they can have a same-sex marriage. This means they can have a wedding ceremony and be legally recognized as married by the government. Same-sex marriage is a way for people of the same sex to show their love and commitment to each other, just like opposite-sex couples do when they get married.