Peter warned the believers about false teachers among them (within the Church). These false teachers distort the Scriptures with heretical interpretations and doctrinal errors (such as denial of Christ or denial of His deity) Destructive/dangerous heresies refer to any wrong teachings that result in the moral and spiritual destruction of those who follow them. By these heresies, people are hurt, harmed, deceived, and destroyed. These false teachers even deny Christ who died on the cross to pay the penalty for their sin. They're motivated by greed/ covetousness. They bring destruction on themselves because God will certainly not let them go unpunished.
I encourage you all to be like the Bereans/to compare everything you hear and read to the Scriptures. "Now the Berean Jews were of more noble character than those in Thessalonica, for they received the message with great eagerness and examined the Scriptures every day to see if what Paul said was true." (Acts 17:11) Guard yourselves against lies and false teaching by familiarizing yourselves with the truth, God's Word! And pray for God's wisdom and discernment.