This operation aims at detecting and correcting mistypes. It tries to correct hostnames (domain names) of emails if there is only one error (missing character like "aompark.com", extra character like "attompark.com" or two adjacent characters with exchanged places like "atmopark.com").
If you don't need to correct mistypes, you can check, that all domains look expected way: three new lists are created.
One list for emails with recognized hostnames, one for absolutely unrecognized and the other for conditionally recognized and corrected (or not corrected, if you check "Check only (don't try to repair detected errors)").
The difficult part in using this operation is specifying the "legitimate" domains. Look carefully at hints, test a couple of times if you want to do it and know, what you do.
The main idea of the interface is specifying groups of 2-nd-3rd level domains after typing their zone (top level domain name). So you write them one per line with a space in the beginning of a line without zone. To check or repair zones, write them without following group of "legitimate" domains: if we "recognize" any domain in zone ".com", we have no reason to check whether it looks like mistype in some "www.xxx.com" or is totally different domain "www.xxxx.com".
The operation creates at most three new lists: "correctable" (in original or repaired form), "good" (with recognized hostname) and "unrecognized": with totally unexpected hostnames, whatever the reason is (too grave error or not enough reasons to enter too much domains to process).
Despite narrowness of correctable errors, some errors may be corrected in different ways. So reviewing "correctable"-s in original form may be a wise decision.
The simplest case in which you may want to employ this operation is if you have a big list of manually entered email addresses with only a few long domain names.