‘The “hirer” means a person engaged in economic activity, public or private, whether or not operating for profit, to whom individuals are supplied, to work temporarily for and under the supervision and direction of that person,’ states the Agency Workers Regulations 2010.
Put more simply, it’s the organisation that hires you (via a recruitment agency) to complete an assignment for them.
The identity of the hirer is not always obvious. For example, the Department for Education (see pages 4-5) confirms that if you work in certain types of maintained schools, the hirer could either be the local authority or the school’s governing body and must be determined on a case-by-case basis. It’s best to check with your agency if you’re not sure.