Definition and example : A head is a piece of information given focus in spoken English by placing it at the front of the utterance rather than incorporating it eg :
That new guy in Accounts, did you know he used to work with David?
Chunking the information in this way aids the listener's understanding, as the topic of the utterance is announced and assimilated first, and the discourse becomes less complex to decode than would be the case if the information was incorporated : Did you know that that new guy in Accounts used to work with David.
Further reading :
McCarthy, M. Exploring Spoken English
Paterson, K. Heads and tails
NB : Don't confuse this use of the term head with either its use in the analysis of grammar, or its use in the analysis of intonation.