Unit test adding to an event

hello I'm a little bit new in c# and WPF. I'm following a tutorial and I found this code which contain unit test with event can you explain it to me :

`[TestClass]
class ObservableObjectTests
{
    [TestMethod]
    public void PropertyChangedEventHandlerIsRaised()
    {
        var obj = new SubObservableObject();

        bool raised = false;

        obj.PropertyChanged += (Sender, e) =>
            {
                Assert.IsTrue(e.PropertyName == "ChangedProperty");
                raised = true;

            };
    }
}`

thank you