Generational sin teaches that you suffer from some kind of judgment or curse because of the sin of your parents or some other ancestor, and that you need to renounce this sin to get freedom from it so you can walk with God.
This is not a biblical teaching. It completely misunderstands the cross, that Jesus has set us free from sin, death, condemnation and the curse. When we trust Christ as Savior we are set free. We are a new creation. We are under no condemnation. We have forgiveness for all our sins. This is our identity in Christ.
We are not under some curse of sin because of our ancestors. There are consequences of sin that hurt us. For example, if your parents were alcoholics or if they got divorced, that affects you. There are consequences. These consequences will usually affect several generations, but there is no judgment or curse that God imposes upon us that we need to renounce or repent of.
This teaching is derived from several passages in the Old Testament that are completely misunderstood. The seminal passage in Exodus 20 clarifies that God visits the iniquity of the fathers on the children of those who hate him. Ezekiel 18 clarifies that no one is punished for the sins of others. Most importantly, generational sin is completely absent from the New Testament. If this is an important part of the spiritual life, where is it found in the Book of Romans? Where is it found in any of Paul’s letters? Where is it found in any of the New Testament? Not one single mention. This teaching is completely contrary to the work of Jesus Christ on the cross to set us free from sin.