What has Christ accomplished by the offering of His body on the cross of Calvary?

He has perfected for all time the saints—those who are being set apart as holy to God. Because He has perfected them for all time, Christ is now waiting in the Presence of God for His (and their) enemies to be made His footstool. The judgment on Satan was finished on the cross. Sin has been done away. The next Divine act will be to remove the presence of sin from the creation.

Does verse 14 mean that it does not matter what we do because we are forgiven anyway?

No. This would be contrary to the writings of the Apostles in the New Testament.

Does verse 14 mean that we no longer are capable of sinning?

No. This would be contrary to the writings of the Apostles in the New Testament as well as to our own experience. All of us come short of the Glory of God in many instances.

Verse 14 of Chapter Ten of Hebrews means that if we continue to press forward in the will of God, availing ourselves of the Word of God, of the body and blood of Christ, and of the power of the Holy Spirit, then the blood of Christ keeps on making a way for us to come boldly before the Presence of the Father so we may obtain mercy and find grace to help in time of need.

Hebrews 10:14 is explained in I John 1:7-9:

But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin (I John 1:7).

If we walk in the light of God's will for us each day, as the Father dwells in light and is that Light, we have fellowship with the Father, and the blood of Christ His Son cleanses us from all sin.

If we say we have no sin, we deceive ourselves, and the truth is not in us (I John 1:8).

If a Christian believer should claim he has no sin he is deceiving himself. He is walking in deception. The truth is not in him.

If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness (I John 1:9).

As long as we do God's will the blood of Christ keeps on cleansing us. As long as we keep repenting and keep walking in Christ, the blood continues to cleanse us from every sin.

The moment the Holy Spirit points out to us a sin in our deeds, our speech, or our thoughts, we are to confess that sin specifically and clearly to God. We are to repent of it and turn away from it with all our determination, asking the Lord to give us strength to overcome it.

The moment we repent and confess the sin, God the Father exercises His faithfulness and His righteousness in forgiving the sin and cleansing us from all unrighteousness.

The Apostle John wrote to us that we should not sin. When we sin we have an Advocate with the Father, the righteous Christ. He is the atoning sacrifice for our sins.

Hebrews 10:14 states that by one offering Christ has perfected forever those who are set apart as holy to the Lord.

As long as we lead a separated life to the Lord, the blood of Christ presents us in righteousness before the Throne of God in Heaven. Let us therefore march forward in the light of God's will, confessing our sins to God when the Holy Spirit makes us aware of them. Let us repent of our sins, receive with thanksgiving the forgiveness and the cleansing, and pray for Divine grace and strength so that we do not commit the same sins again. (from A Study Guide for the Book of Hebrews)