means,

That, if you are born again, and abiding, and walking "in the Spirit", you are not controlled and regulated by your flesh, or even God's laws, but are, rather, in Christ, in His strength, fulfilling God's laws. No longer "servants to sin", as we "... sit together in heavenly places in Christ Jesus", we now instead become "the righteousness of God in Him", daily overcoming our flesh, sin, and worldliness in his strength.

This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh. For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would. But if ye be led of the Spirit, ye are not under the law.
Galatians 5:16-18

References:

"Born Again": John 3:3-8 and 1 Peter 1:22-25

"The righteousness of God in Him [Christ]": 2 Corinthians 5:21

"...Sit together in heavenly places in Christ Jesus...": Ephesians 2:4-6

"Servants to sin": Romans Chapter 6