custom properties or special attribute or is this a pattern?

custom properties or special attribute or is this a pattern?

I'am trying to figure out what programming technique they are using here.
As you may see "Class AAA" has a type-class property called "MessageInfo".
I need to know if this is a "custom property" or a kind of special
attribute.
I tried by doing research and reading different books, and I'm still very
confused.
public class AAA
{
public BBB MessageInfo { get; set; }
object.MessageInfo.text = "xxxxx";
}
public class BBB
{
// text here...
}