For example, take a simple sentence such as:
John saw Mary in the park
This sentence can be broken down into three distinct parts - or 'phrases'. The first is 'John'. And what did John do? He 'saw Mary'. Where did he see her? 'In the park'.
John saw Mary in the park
These three 'phrases' have distinct grammatical functions. The first ('John') is clearly determined by the fact that it is a noun. It is a 'noun phrase', which we will label 'NP'.
John saw Mary in the park
NP
'saw Mary' is composed of a verb ('saw') and another noun ('Mary'). We know that these two words go together because if we try and split them up then the sentence changes meaning.
To say 'Mary saw John' would be to mean something different. To say 'John saw the park in Mary' would be gibberish. 'Mary' has to stay with 'saw', and of these two words the verb 'saw' is clearly the more important because it defines what John is doing - so this phrase is a verb phrase (VP), comprised of a verb (V) and a noun (N):
John saw Mary in the park
NP
VP
V
N