Typically agency work means you work for a recruitment agency to do temporary work for their clients. In the UK, agency workers have certain rights under the Agency Workers Regulations.
The agencies act as a go-between for businesses and workers. The business uses the agency to find workers for specific tasks or projects, and the worker uses the agency to find fast and flexible employment.
The assumption with many agencies is that the worker is available for hire on a short-term basis. This can be for a day, a week, or even just a few hours. The nature of the work can be varied as well—from manual labour to office work and everything in between.