It refers to the fact that a new creation will bear good fruit, have good works, etc. as an expression of his/her faith and the indwelling of the Holy Spirit. According to Jesus, if you see someone bearing bad fruit, it's a good sign of unregeneracy. N.b. Bearing bad fruit doesn't mean just struggling with sin sometimes - all Christians do. It is when you are obviously a thorn bush and not a fig tree.
Does that help a little?
"I wish it were in my power, after this day, to cry down all love but the love of Christ, and to cry down all gods but Christ, all saviours but Christ, all well-beloveds but Christ, and all soul-suitors and love-beggars but Christ." -Samuel Rutherford, Letter CCIII. to William Gordon at Kenmure