Look at the following sentence :
a) Dogs should be kept on leads.
The highlighted word dogs is a noun, acting as subject of the sentence. It can be expanded...
b) All dogs should be kept on leads.
c) All large dogs should be kept on leads.
d) All the dogs in your charge should be kept on leads.
or, in all these cases, reduced to a pronoun if the referent is clear from the context.
e) They should be kept on leads.
All these highlighted examples are therefore equivalent. They form a noun phrase within the sentence. The head of the noun phrase is the word that cannot be omitted - in this case dogs or they - the noun or pronoun itself. When there are other elements they are referred to as the pre-head or post head :
All the (pre-head) dogs (head) in your charge (post head)...
And the pre- or post- heads may themselves consist of another type of phrase - so here, for examples, in your charge is a prepositional phrase, with the preposition in as head and your charge as post head. This post head is in turn a noun phrase, made up of pre-head your and head charge.
Phrases (in italics with the head underlined) may be :
- Noun phrases : The sun shone very brightly / I don't like his films
- Verb phrases : David burst into the room / I don't like his films
- Prepositional phrases : David burst into the room
- Adverb phrases : The sun was shining very brightly
- Adjective phrases : You look very nice
For a clear and much more detailed explanation of phrases and phrase structure, with activities to check your understanding see :
University College London Introducing Phrases
which takes you step by step through the analysis of the different types of phrase. At the end of each page you'll find a link to the next section.