Here we have reached the end of definitions (what things are) in book one as well as the axioms (how things relate). This is the first of the propositions - a series of statements made in the form of a geometric proof that logically extrapolates the definitions and axioms.
Another rendering to proposition 1 might be "A substance is prior in nature to its states." Basically, I understand Spinoza to describe a universe of immanent causality rather than transitive or transcendent causality. This distinction is that immanent causality is the way in which a circle causes roundness. The roundness is caused by the definition of or nature of the circle. Such causality is different than the watchmaker who makes a watch. The watchmaker has a transitive or transcendent causality. The watchmaker makes the watch and is separate from the watch itself. The circle is prior to roundness and can only be different if the definition of circle changes. Thus, the importance of definitions 3 and 5. He refers us to the definitions of substantia or "substance" (I.D3) and substantiae affectiones or "modifications of substance" (I.D5). The clear contrast between these two definitions is that "substance is in itself" and "is conceived through itself," while "modifications of substance are in another through which they are also conceived." From this distinction, substance is independent, while modifications of substance (or affections or states or modes) are dependent. In the same sense (although on a fundamentally different level and just for clarity's sake), "roundness" might be understood as a dependent modification while a "circle" might be understood (not Spinozistically, of course) as an independent concept. In this way substance has priority over its modification in the same way as a circle has priority over roundness. (Of course the example here fails in terms of an essence that involves existence. A circle could simply not exist - unlike substance.)
Substantia prior est natura suis affectionibus.
Translated as,
Substance is by Nature prior to its modifications.
Also translated as,
A substance is prior in nature to its affections.
Demonstratio: Patet ex definitione 3 et 5.
Dem: This is evident from I.D3 and I.D5.