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?
Austin | Christ Covenant Reformed Presbyterian Church (RPCGA) | Texas
"He was immovable in all times, so that they who in the midst of many opinions have lost the view of true religion, may return to him and find it."
-John Geree, The Character of an Old English Puritan, or Non-Conformist (1646)