You can listen to John Piper talk about the meaning of justification by faith alone in the audio link above. We also offer the following summary of John Piper's view.

John Piper holds to the historic, Protestant doctrine of justification by faith alone, which can be summarized in the following four points:

1) The sole ground of our justification is the righteousness of God, expressed in the alien, imputed, active obedience of Christ, climaxing in his sin-bearing, substitutionary death.

2) Faith alone is the sole means of justification. In other words, it is faith only, and not our deeds in any way (whether the external manifestation or the internal God-glorifying motive behind them), that connect us savingly to Jesus Christ.

3) Faith is distinct from its fruit, the obedience of faith, yet faith is of such a nature that it must and will produce love for people and a life of genuine, though imperfect, holiness in this world. Therefore, as the Westminster Confession of Faith (11.2) says, the faith that alone justifies (as the instrument which unites us to Christ, not as the ground or content of our justifying righteousness) is never alone;

4) Therefore, this reality of forensic righteousness, which is imputed to us on the first act of saving faith (as the seed of subsequent persevering faith), is different from transformative sanctification, which is imparted by the work of the Holy Spirit through faith in future grace.