It refers to a person who is a knowledgeable and discerning judge of a particular art, activity, or type of food or drink.
For example:
* "He is a connoisseur of fine wines."
* "She is a connoisseur of classical music."
While "connoisseur" is not a verb itself, you can use it to form verbs like "to connoisseur," meaning "to be a connoisseur of." However, this usage is quite uncommon.