It is an organised event, where potential job seekers, and potential employers can showcase themselves, and meet. Companies who want to hire new employees set up booths that give information about their company. Potential employees can go around the displays, to look at what is on offer.
They would often be run by schools or universities, for new graduates, but potentially in other cases, such as if a factory was going to close down, and make their workers redundant. The could set up a job fair so that other companies in the area could see if they wanted to employ some of the workers.