This is a key aspect of writing (and speech). Whilst the grammar of a sentence is what makes its words and phrases appear coherent and unified (i.e. hold together and be leading to a single central idea) - and one that helps when you are trying to work out meaning from a text. It's in the nature of the way literary writing is constructed that its meanings are all connected and leading in one particular direction (i.e. the meaning is unified). So... the opening chapter of, say, Steinbeck's short novel, Of Mice and Men has a meaning of its own, for sure - but that meaning will also be a part of the whole and so must be contributing in important ways to the overall story and its themes.
Similarly, each line and each image of a poem or play is contributing to its overall meaning. This means that if you decide a line of poetry, for example, has a particular meaning, you can only be right if you can show how that meaning contributes to the overall meaning of the poem.