The noun mulatto is an old-fashioned way to refer to someone with one black parent and one white parent. It was common to describe a mixed race person as a mulatto during the period of slavery in the United States.
If you use the word mulatto to describe someone of mixed race, most people will know what you mean, but they may also be offended. People of mixed race are much more likely to describe themselves as biracial than mulatto these days. The Spanish root word, mulato, is used to mean "of mixed breed," although it literally means "young mule," from mulo, or "mule."