Well, it is the best phrase I can think of to describe art that draws those who recieve it into relationship with God. There is creation itself which, according to the Bible, "declares the glory of God." So that means all of creation is "missional art." If we are receptive to God's art, if we "open our eyes" and hearts, then we can know a great deal about The One who created it.
I think most of the art that people create has a purpose, perhaps all of it does. Even when little children draw what looks like random lines they are "trying to get something out." They are communicating.
But, when I use the phrase "Missional Art" I am referring to art created by someone who knows God and who, through the creative process, intentionally or unintentionally, communicates something about God. Some of the most effective Missional Art is not symbolic (such as a painting of the cross). The rock band U2 is a great example of outstanding Missional Art that is subtle but powerfully missional.
Help me expand on this....