A connoisseur is someone who knows a lot about something and likes it a lot too. For example, if someone knows a lot about cars and loves them so much, we can call them a car connoisseur.
Think of it like this: If you really love candy and you can tell the difference between different types of candy because you have tasted a lot of them before, you can be called a candy connoisseur.
Same goes for different types of foods like cheese, wine or chocolate. They taste different from each other and some people can recognize the difference because they have the skill of tasting things like an expert. These people are called connoisseurs of these foods.
In a nutshell, a connoisseur is a person that has a lot of knowledge and love about something and knows it like the back of their hand.