HERMS stands for Heat Exchanged Recirculating Mash System.
The HERMS allows the brewer to recirculate the wort from the mash tun, through a heat exchanger and back into the mash tun. The temperature of the recirculated wort is controlled by the heat exchanger which, as a result, controls the temperature of the mash very precisely. The added benefit is that the action of recirculation clarifies the wort by using the grain bed as a filter.