A collective noun names one thing that has many items included in it.
The word group is a collective noun. A group is one thing with many things inside of it
You might have a group of kids.
You might have a group of teachers.
You might have a group of friends.
We call the word group a collective noun because the noun is a word for something that has many pieces inside of it.
Here are more examples of a collective noun:
team
class
family
troop
band
choir
orchestra
crowd
gang
Each of these words names a thing that has many different people included in it.
Look at these collective nouns. They each name something with many different things inside.
collection (of coins or other objects)
deck (of cards)
fleet (of taxis or other vehicles)
pair (of shoes or socks)
bouquet (of flowers)
bundle (of sticks)
clump (of bushes)
pack (of gum)
A collective noun names something with many parts inside of it.