Thought
Thought is one of the two attributes of substance we can know. It is the mental aspect of reality.
In depth
Under the attribute of Thought, substance expresses itself as ideas, concepts, and mental states. Every physical thing has a corresponding idea in God's infinite intellect. The human mind is the idea of the human body. Thought is not produced by matter — it is a parallel expression of the same reality that matter also expresses.
Why it matters
This makes mind fundamental to reality, not an accidental byproduct of brains. Panpsychism aside, Spinoza is saying that the mental and the physical are always co-present, just described differently.