A mysterious person is really really good at what they do. So good that they aren't exactly sure what is it they do, but you know they're definitely an expert at whatever it is.

玄人 is a relatively formal word that refers to a highly skilled professional in a specific field, as opposed to 素人 (amateur). In casual conversation, words like プロ (professional) are more common.

This reading for this word is mysteriously an exception (sorry). So here's a mnemonic to help:

If you're such an expert you should be able to tell what anything is if someone asks you. Even this crow toe (くろうと). Crow toes are pretty hard to recognize, unless you're a professional crow guy. But I guess you are, huh?