Spoken grammar: lexico-grammatical features of conversation highlighted in corpus research.
A ‘head’ is a bit of language (normally a noun phrase) that’s been taken from its usual position in a sentence, placed at the beginning, and then represented in the second half of the sentence by a co-referential pronoun:
What are the people over there waiting for? (normal word order in a question)
The people over there (head), what are they (pronoun) waiting for?
We tend to use heads to emphasise (or ‘point at’) what is important by mentioning it first, thus separating it from the rest of the sentence, and to make complex utterances easier to process by breaking them into two parts:
Are those new jobs you’re applying for part-time or full-time?
Those new jobs you’re applying for, are they part-time or full-time?
As in the examples above, utterances with heads are often questions, but they don’t have to be:
That tall guy, he’s looking at you.
Jessica, you can ask her anything you like about films, and she'll know the answer.
A variation on the form – by changing the subject and using a possessive pronoun – is also quite common:
One of my friends at college, his uncle has an apartment in Berlin.
(Perhaps, since we don’t know what ‘punctuation’ a speaker is using, you could argue that heads, in the way I’ve described them, don’t necessarily exist within a single sentence: ‘That tall guy, he’s looking at you’, for example, could be transcribed as ‘That tall guy. He’s looking at you’ – in other words, a verbless first sentence, followed by a regular sentence.)