See Michael Della Rocca (editor), The Oxford Handbook Of Spinoza (Oxford University Press,
2018), Chapter 5 The Building Blocks of Spinoza’s Metaphysics: Substance, Attributes, and Modes by Yitzhak Y. Melamed.
One of the major questions of metaphysics throughout its history has been: What is? Spinoza has an astonishingly brief answer to this question: God. All that is, is just God (and his qualities).
Spinoza’s God has infinitely many qualities that constitute, or are conceived as constituting, his essence, while the other qualities of Spinoza’s God, though not constituting God’s essence, follow necessarily from God’s essence. Spinoza calls the former “Attributes [attributa]” and the latter “Modes [modi].”
Thus, there is substance: God, with qualities constituting his essence (attributes).
Def.1.IV. By attribute, I mean that which the intellect perceives as constituting the essence of substance.
Def.1.V. By mode, I mean the modifications of substance, or that which exists in, and is conceived through, something other than itself.
Thinking and extension are attributes of God [Prop.2.I and 2.II].
Prop. 2.VI. The modes of any given attribute are caused by God, in so far as he is considered through the attribute of which they are modes, and not in so far as he is considered through any other attribute.
Bodies are modes of extension [Def.1.I] and ideas are modes of thinking [Ax.2.III].
In conclusion, ideas are modes of thinking that in turn is an attribute of substance [God]: we can say that ideas are waves on the "surface" of thinking, in the same way as bodies are waves on the "surface" of extension and both, thinking and extension, are the dual aspects of unique substance.
Prop. 2.XI. The first element, which constitutes the actual being of the human mind, is the idea of some particular thing actually existing.
we have that ideas [Def.2.III "the mental conception which is formed by the mind"] of existing things ["particular thing actually existing"] are modes of [constitute] the mind of an existing human.