A noun is a naming word, as you know: The girls kicked the ball. The two nouns are girls, and ball.
A noun phrase is a group of words that plays the role of a noun in a sentence. It answers the question “who/whom or what?” For example: The group of girls kicked the home-made ball. So in this sentence, The group of girls and the home-made ball are noun phrases.
Another example: I like all the people in my class. (Whom do you like?) “All the people in my class” is a noun phrase. Here are two more examples:
The barking dog is driving me crazy. (What is driving you crazy?)
The soccer players are ready for the game. (Who are ready for the game?)