It has to do with nations allowing for their war criminals to be tried in international tribunals. I know that the U.S and Israel are among the few nations that did not sign. Sorry if this is the wrong place. I went through the subreddit rules, but wasn't totally sure If I landed in the right place.
Are you thinking of the Rome Statute of the International Criminal Court? There are numerous countries that signed but haven't ratified it. Of those, Israel, Sudan, and the U. S. are the three states which have unsigned the treaty.
In addition, there are 41 countries which either did not sign the treaty before the 2000 deadline, or did not exist before the deadline (e. g. South Sudan).
Here's a short column by the former U. S. ambassador who signed the Rome Statute, which sheds some light on the political situation that resulted in the U. S. unsigning.