Definite atonement is a theological term that’s often called limited atonement, or better described as particular redemption. In a nutshell it is the belief that the Lord Jesus Christ offered Himself as a sacrifice upon the cross with a particular people in view called the elect. That He died with redemptive effect specifically for them. This view properly presents the death of Christ as a success.
Take Judas Iscariot as an example, Did Christ die for him? If so, then He suffered the wrath of God for the sins of Judas, and yet Judas is in hell, which means that even though Christ apparently paid for the sins of Judas, God is still demanding that Judas pay for them as well. This makes God unjust.
Furthermore, Paul says in Ephesians 5:25 that “Christ also loved the church, and gave himself for it.” i.e. He gave Himself specifically for the church.
So definite atonement is the correct belief that Christ secured salvation for all for whom He died. None of this negates your personal responsibility to receive Christ yourself. Have you done that?
Armen