Substances have attributes.
But Descartes thinks that each substance has one attribute which is fundamental
to it.
Descartes calls such a property a 'principle attribute' : one which 'constitutes
... [a substance's] nature and essence, and to which all its other properties
are referred (1.63) w., p.19. Cottingham p.177.
'A substance may indeed be known through any attribute at all; but each substance has one principal property which constitutes its nature and essence. Thus extension in length, breadth and depth constitutes the nature of corporeal substance; and thought constitutes the nature of thinking substance.' Principles, 53; Cottingham p.177.
Each substance has one of these principle attributes: extension for corporeal
substance, thought for thinking substance.
Expanding the claim that the principle attribute of corporeal substance is extension Descartes says that everything else that is true of corporeal substance is merely a mode of an extended thing.
The characteristically Cartesian term 'mode' is Descartes term for those properties which are 'referred' to the principle property.
Substance < Principle property
^
Mode
The principle attribute of thinking substance is thought: and all other attributes
of the mind are modes of thinking.