Have a look in the OT to see what God commanded people to do when they sinned in order to be forgiven.
They had to approach His priests in the Temple, and bring an animal as a sacrificial offering (a visible sign of their atonement for the wrong they’d done). Note the animal had to be perfect - without any blemish.
The priest then slaughtered the animal on the person’s behalf, praying for forgiveness of that person’s sins, and burnt it on God’s altar. Problems being that a) this process had to be repeated on each occasion that someone wanted to atone, and b) priests were limited in their agency, since they were flawed human beings.
Christ was the only perfect Priest of God since a) he was sinless himself and b) he was eternal, thus the only one capable of offering for ALL sins for ALL time.
He was also the only eternally acceptable sacrificial victim as well - again, since a) He was sinless and without blemish and b) He was eternal.
And so, by both officiating as priest and offering Himself as victim, He made the once-and-only perfect sacrifice that atoned for mankind’s sins throughout human history and reconciled us to the Father.
Hope this makes sense.