ので expresses an objective reason for an action. The clause providing a reason is before ので, and the clause after is the result. ので places emphasis on the effect. It is similar in meaning to “since” or “because”, but by its objectiveness, it can also be considered to mean “by the fact that” or “from the fact that”.
When ので follows a non-conjugating word (a noun or な adjective), it is written なので. When following a conjugating word (verb or い adjective), ので by itself is used.