The covenant of works refers to the initial arrangement God made with Adam in the Garden of Eden, wherein God promised eternal life upon condition of perfect, personal obedience and threatened death upon disobedience. This federal arrangement established Adam as the representative of all humanity, meaning his failure resulted in the fall of all his posterity into sin and misery.
The 1689 LBCF Ch.7 §1 explains that the immense distance between the Creator and the creature meant that humanity could only attain eternal life through God's voluntary condescension in a covenant. Regarding the specific covenant of works, 1689 LBCF Ch.19 §1 details that God gave Adam a law of universal obedience, alongside the prohibition concerning the tree of the knowledge of good and evil. This bound Adam and all his posterity to perfect, entire, and perpetual obedience, with life promised for success and death threatened for the breach of that law.
Because of our federal head, Adam, we are all born into a state of sin and misery. However, we rejoice that God does not leave all mankind to perish. As noted in Westminster Larger Catechism Q.30, out of His mere love and mercy, God delivers His elect from the ruin of the first covenant and brings them into the estate of salvation through the second covenant, the Covenant of Grace. This grace is found only in the Lord Jesus Christ, the second Adam, who rendered the perfect obedience we could not, satisfying the law in our place. As the Scripture says, Romans 5:18-19.