What is the [fact] attribute?

I'm sure this is super simple, but I'm clearly not Googling the right thing. In several unit test related blogs, I've seen the attribute [fact] given to several methods, but I can't seem to figure out what this means. Example:

public class SomeClass
{
    [Fact]
    public void Foo()
    {

    }

    [Fact]
    public void Bar()
    {

    }
}

It belongs to the xUnit unit testing framework.

It says that the method is a unit test.