What is DNS TXT record "mailru-verification"?

Solution 1:

It's domain verification for service PostMaster.mail.ru. There you can get some statistics about your letters delivery, spam, problems, configure feedback loop and so on. Quite handsome.

If you have big audience with @mail.ru addresses you may find it useful for you. It has an interface in English too.

Solution 2:

I suspect it's used for domain ownership verification when signing up for some SaaS-style service where you're allowed to use your own domain name.

This is fairly common practice to thwart abuse, and in such cases no one but that service provider is expected to actually make use of the record.

Presumably this one is for some service provided by mail.ru, based on the mailru bit in the value.