phrase meaning - What does 'just so you know' mean?

“just so you know” (Urban Dictionary)

What does this mean? And what grammatical role does “so” have?

Answer

Just so you know is a pretty ordinary construction, as such things go in English.

Just = "only"
So is the ordinary adverb, employed in the sense "in order that"
*You know" = "you know", "you are aware of"

So:

I provide this information (without being asked) in order that you may be aware of it and thus be spared embarrassment or inadvertent error.