This phrase can mainly be attributed to Ed Silvoso, great friend/mentor and longtime leader of Harvest Evangelism as well as, more recently, an International Trans-formation Network. I've included just one of his books, although I've read most of what he's written in the past couple of decades. Before he started talking and writing about discipled nations he was big into prayer evangelism and city-reaching, through his initial work in his home-country, Argentina.
One of the key things Ed was pointing out early on, was that "our work is our worship" so that the bulk of our worship is not what we do, rather as the church gathered on Sunday morning, but what we all tend to do nine-to-five during our work week. This is a biblical ideal which completely alters our view of work, that it is not a result of the fall, but rather a singificant part of our being made in his image, and therefore, as a few other writers have put it, our marketplace must be a significant arena in which the values of the kingdom of God are once again restored. This is part of a massive corrective, in which we cease to separate reality into the sacred (i.e. full-time ministry, church buildings, etc.) and secular (what we do for the bulk of our work week and in our liezure time) and realize that Christ's kingdom reaches into the furthest corners of our human life.
When we talk of obedient nations this is clearly a signficant part of our larger task, and Ed is one teacher/practitioner who has spent years working at this level, to partner with marketplace leaders--to actually see the values of the Kingdom of God established as much in the marketplace as in our routinized holy places like churches and mission agencies.
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Transformation of marketplace requires that change of perspective, in which we understand that Christ's redemptive work achieved victory over the forces of darkness, that he has already redeemed them and, in Ed's words, they must now be reclaimed for the intended purposes and glory of God. If nations are actually taught to obey Christ, then the marketplace will be a flashpoint, a classic proving ground for this transformation to take hold--at our larger collection of the human family.