The chevrons are used to denote an attribute. There are many more attributes than just MarshalAs and they are used for many and varied purposes. For instance, the way you see properties displayed in the Properties window for a control is determined by several attributes. For instance, the Text property of a control is listed in the Appearance section of the Properties window, which is achieved by declaring the property like this:
vb.net Code:
  1. <Category("Appearance")>
  2. Public Property Text() As String
  3.     '...
  4. End Property
Attributes are metadata, i.e. they are not part of the type or member itself but they are information about the type or member. In your example, the attribute determines how that member is mapped to and from unmanaged code.