Necessity
Everything that happens follows necessarily from the nature of God/Nature. There is no contingency, no "could have been otherwise."
In depth
Spinoza is a strict determinist. Every event, thought, and action is determined by prior causes, which are themselves determined, all the way back to the nature of substance itself. Nothing is contingent — things appear contingent only because we are ignorant of their causes. Even your choices are determined by your nature and circumstances.
Why it matters
This sounds like it eliminates freedom, but Spinoza redefines freedom as acting from your own nature rather than being compelled by external forces. Necessity is not oppression — it is the structure of reality. Understanding it is the first step toward the only real freedom available.
Source references
- Ethics I.P29-P33