1. For that reason; therefore: This is the easiest way to get there, so don't argue. 2. With the result or consequence that: He failed to appear, so we went on without him. 3. With the purpose that: I stayed.
The conjunction so (often followed by that) introduces clauses both of purpose (We ordered our tickets early so that we could get good seats) and of result (The river had frozen during the night so people.
Used to preface a remark or signal a new subject. So what happened here? So I'm going to the store to buy some milk.