Question

1. I took a shower so I felt refreshed.

2. I took a shower so that I felt refreshed.

3. I took a shower, so I felt refrewhed.

(What does #1 mean, #2 or #3? In Sentence 1, there is no 'comma' before 'so.' In Sentence 1, we cannot see 'that' after 'so.')

Answer 1

All three mean about the same thing. The second clause in each is the result of the main clause.

"so" or "so that" is the subordinating conjunction that give the dependent clause its "result" meaning.
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions