It means that we are God's and He is ours. We are one in Him. And when we are one, we must share in His suffering. No suffering will amount to what Jesus went through, but we have our share. It ends in verse 18 that any suffering we go through now will be taken into account with God...and He will reward us for being faithful to Him throughout the suffering.
First you must learn to qoute scripture in context.
This scripture is talking about being Born of the Spirit The Holy Spirit.
12 Therefore, brethren, we are debtors—not to the flesh, to live according to the flesh. 13 For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live. 14 For as many as are led by the Spirit of God, these are sons of God. 15 For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, “Abba, Father.” 16 The Spirit Himself bears witness with our spirit that we are children of God, 17 and if children, then heirs—heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together.
This scripture is talking about the price you must be willing to pay for following after Christ compare to the glory that will be revealed in the afterlife.
This scripture has reference to spirit anointed Christians who look forward to life in heaven as Kings and Priests with Christ. Rev 20:6 " Blessed and holy are those who share in the First Resurrection. For them the Second Death holds no terrors, for they will be priests of God and of Christ; and shall reign with him a thousand years."(Living Bible). Verses 16 and 17 show that God's spirit makes it plain to them that they are meant for heavenly life. They have the firm conviction that they are God's spiritual sons and that when they die , they will be resurrected to heavenly glory, as Jesus was. Verse 18 shows that despite what they must suffer, that have such a strong heavenly hope, that they willingly sacrifice all earthly prospects and attachments.