For me plain is - what is pleasing to God. It is allowing every aspect of life to be organized around the love of God - a way that shows the Gospel power and promise.
The function I think is - not putting any limit on the way God's power may come to dwell plentifully in us - answering God's call. It is a path of only having the possessions that God's peace and justice show us is our right share; living in order to bear witness to the precious gift of grace that God is bringing to birth in the world. Gospel order in the material things of our lives.
Plain is - simple, hardwearing, beautiful-in-God's-way, justice-making, stands outside the fashions of media culture, honours the body as the temple of the Holy Spirit, and as the site of our prayers and our transformation into God's co-workers, it's ecologically-appropriate, and tends towards using God's love, loving relationships of connection between humans, and freely-offered labour, to reduce and replace exploitative monetary relationships and fossil-fueled chains of action.
I'm not so much up with the history and origins ... Let's see if Isabel is around for some of that, she has a lot of knowledge in that area.