So this makes sense. Google tells me that IFR is Infection Fatality Rate, which I can see would be very much a retrospective calculation. (I comfort myself that there will be people writing dissertations about this stuff in the future. That means we HAVE a future.) CFR is Case Fatality Rate which currently is much higher and what is being tracked on Worldometer.
This is my summary of a post on a blog called Virolgy Blog by Vincent Racaniello, who seems to be a professor at Columbia University. The post, written in April, seems relatively straightforward, though the numbers he cites didn't hold up very well considering what we are seeing now. But then, he probably didn't anticipate the many decisions that have been made on a personal, local and national level about how much risk to vulnerable populations is acceptable. He also has some suggestions about how to test to get a better handle on the likely IFR, but I don't know how much of that has been done.
Infection Fatality Rate – A Critical Missing Piece for Managing Covid-19
Here is a more recent discussion of IFR on the site nature.com. It is not willing to commit to a number and is a good discussion from mid-June of some of the problems in determining it.
How deadly is the coronavirus? Scientists are close to an answer