Thought and Extension are attributes we know
Formal Statement
Thought is an attribute of God (II.P1): God is a thinking thing. Extension is an attribute of God (II.P2): God is an extended thing. These are the two attributes of substance that the human intellect can perceive. Each is conceived through itself (I.P10), so neither is reducible to the other — yet both express the same single substance.
In Plain Language
Part I built the architecture: one substance, infinite attributes, all particular things as modes. Now Part II names the two attributes we actually have access to: Thought and Extension — mind-stuff and matter-stuff. Because each attribute is conceived through itself (P10), you cannot explain thought by extension or extension by thought. But because they are attributes of the same substance, every event in the order of extension has a parallel in the order of thought, and vice versa. This is the foundation of Spinoza's solution to the mind-body problem: not interaction, but identity under two descriptions.
Why This Follows
I.P10 (gs-11) proved that each attribute is conceived through itself. I.P11 (gs-12) proved God necessarily exists with all attributes. I.P25 Cor. (gs-16) showed individual things are modifications of God's attributes. Now II.P1 and II.P2 identify Thought and Extension as two such attributes: particular thoughts are modes of God qua thinking, and particular bodies are modes of God qua extended.
Thought and Extension are two known attributes of the one substance — conceptually independent but ontologically identical.
Connected Concepts
If Thought and Extension are two descriptions of one reality, why does it feel so different to have a thought versus to bump into a table?
Path complete!
You now understand Spinoza's metaphysical foundation: one substance, infinite attributes, and everything as a mode. This is the ground floor for everything else in the Ethics.