A proper noun is a name of a specific person, place, or thing. For example, "John Smith," "New York City," and "the Empire State Building" are all proper nouns.
A common noun is a name of a general category of people, places, or things. For example, "man," "city," and "building" are all common nouns.
The word "flute" is a common noun because it refers to a general category of wind instruments. There are many different types of flutes, such as the piccolo flute, the alto flute, and the bass flute.