The simplest noun group consists of a noun and a pointer.
Lee was standing against a tree.
Pointers tell us more precisely which thing or whose thing we are talking about. They point to the thing we’re talking about.
Take a look at these examples.
Lee was standing against the snowman.
Lee was standing against his motorbike.
Lee was standing against his grandmother’s statue.
There are all sorts of pointers: a, the, his, her, my, their, our, that, those, Jenny's, the Empire's...
Pointers are often called determiners in other grammars.
We use the term pointers because we think it's easier to understand and remember, but they are roughly the same thing.