It's a prepositional phrase that literally translates to "of someone", and it is used to indicate possession, origin, or relationship.
Here are some examples:
* El libro de alguien: Someone's book (possession)
* Un regalo de alguien: A gift from someone (origin)
* Un amigo de alguien: A friend of someone (relationship)
It's important to note that "de alguien" is always used with a singular noun, even if you're referring to multiple people.
For example, "the books of some people" would be "los libros de algunas personas."