Sentences are made up of smaller phrases. There are several difference types of phrase that can be used in a sentence, but the two phrases which must be used in a sentence for it to make sense are a noun phrase and a verb phrase.
In a phrase, we must have a word which is called the head. This is the core of the phrase, what the phrase can’t exist without. So in a phrase like ‘the dog’ or ‘ran far away’, in the first phrase ‘dog’ is the head because it is the main part of the phrase, and in the second phrase ‘ran’ is the head because it is essential for the phrase to exist. We can have ‘dog ran’, which isn’t grammatical, but this still makes sense since we can understand that the dog ran. But we can’t have ‘the far away’, this makes no sense to us!