(1 John 2:2)"He is the propitiation for our sins, and not for ours only but also for the sins of the whole world." (ESV)
(1 John 2:2)"He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world." (NIV)
The major difference between these two translations in this verse is one word or phrase. The ESV has "propitiation" while the NIV has "atoning sacrifice." The phrase "atoning sacrifice" has the meaning to make amends, to pay for sins. The term "propitiation" has a fuller meaning. It means the payment for sins, but also includes turning away the wrath of God and turning it to favor.
The NIV rendering of "atoning sacrifice" does not provide the reader with the full meaning. While the ESV rendering of "propitiation" does provide the full meaning for the reader. Christ is the propitiation for our sins, for all believers in the whole world. Our sins incur the wrath of a holy God. The sacrifice of Jesus on the cross turns away the wrath of God from us and turns it to favor for us. Because God's wrath has been propitiated or satisfied we can now have forgiveness.
The more literal translations such as the ESV, NASB, and NKJV all have propitiation. The less literal translations such as the NIV, CSB, and NLT have atoning sacrifice or something similar.