On the character sheet, under the Saving Throws section, there is "Corruption" listed. I can fill in the "proficiency" bubble or write a modifier in.
My question is: What am I supposed to use that part of the sheet to keep track of? Resisting Corruption is done by making a Wisdom saving throw, which is already on the sheet, so I don't understand why there's a separate saving throw proficiency for "Corruption" saving throws, whatever those are.
A user in the /r/AiME subreddit, megabuster44, had essentially the same question as you.
Mac_na_hEaglaise's comment explains (emphasis mine):
I’ll break it down as I understand it:
All “Corruption Checks” are Wisdom Saving Throws (usually DC 15).
Not all Wisdom Saving Throws are Corruption Checks.
You can be proficient in Wisdom Saving Throws (as a Scholar is), which means you are proficient in Corruption Checks. Roll as normal for a Wisdom Saving Throw.
You can be proficient in Corruption Checks (more accurately, any Wisdom Saving Throws that risks gaining shadow), even though you may not be proficient in Wisdom Saving Throws as a whole. High Elves of Rivendell get this, and the Dúnedain’s Virtue “Dauntless Guardians - Strength of Will” gives this proficiency in a more limited, but more powerful way. An item might also give such a proficiency. You would add your proficiency bonus if the conditions apply (it risks gaining shadow, or even has a narrower focus for Dauntless Guardians), but not if it’s a different kind of Wisdom Saving Throw and you don’t have overall proficiency in them.
The Dúnedain Cultural Virtue "Dauntless Guardians" has the 2nd-tier benefit (learned as an undertaking during a Fellowship Phase after selecting the virtue) Strength of Will (Guia do Jogadorp. 104):
When an Undead creature forces you to make a Wisdom saving throw against Corruption or any effect that may cause the Frightened condition, you may add your proficiency bonus to the check. If you roll either a 19 or 20 to pass the check, you can allow a number of companions equal to your Wisdom modifier to pass their own checks automatically.
High Elves of Rivendell have the Elf-wise trait (Rivendell Region Guidep. 117):
You have fought against the Dark Lord for years uncounted, and will continue to strive against the Enemy. You are proficient in Corruption checks. A Corruption check is any Wisdom saving throw that risks 1 or more Shadow points for failure.
In addition, as RobinSmallburrow's comment and megabuster44's reply note, Hobbits of the Shire have the Resilient trait (Guia do Jogadorp. 47):
You have advantage on saving throws against Corruption.
Saving throws against Corruption are indeed Wisdom saving throws, but the official character sheet lists them separately in order to help distinguish when one's proficiency bonus applies to such saves based on character or item benefits that apply specifically to saves against Corruption.