If you look up the word "propitiation" in the ESV, it only appears four times. Other translations have "atoning sacrifice" or some other usage of words because no one says big words like that anymore. Granted propitiation is not on my list of words to say in day to day conversation, but this is an important biblical word.
Wayne Grudem defines propitiation as "A sacrifice that bears God's wrath to the end and in doing so changes God's wrath towards us in favor." He goes on to say that some theologians cannot comprehend that God is a God of wrath because God is love. It is true that God is love, but He cannot let sin go unpunished.
Many pastors will not teach why propitiation because it is too complex whiles water down the word. There are still pastors and theologians that would deny propitiation because of the idea that God laid upon Jesus the wrath of God in our place for our sins. John Murray, in his book, Redemption Accomplished and Applied, said:
To deny propitiation is to undermine the nature of the atonement as the vicarious endurance of the penalty of sin. In a word, it is to deny substitutionary atonement. To glory in the cross is to glory in Christ as the propitiatory sacrifice once offered, as the abiding propitiatory, and as the one who embodies in himself for ever all the propitiatory efficacy of the propitiation once for all accomplished.
To deny propitiation is to deny to the finished work of Christ on the cross. To deny propitiation is to deny God's provision for our sins to be atoned for. This is why studying theology is important so we know what God did through Christ on the cross. To know what propitiation is to stand in awe of what God has done through Christ.