Complex sentence: Attributes are data about the window, such as its size, position, background color, etc. Properties are strings attached to a window, and have no meaning at the level of the X protocol.
Split into simpler sentences: Attributes are data about the window, such as its size, position, background color, etc. Properties are pieces of data that are attached to a window. Contrary to attributes, properties have no meaning at the level of the X protocol.