The moderator of a usability test walks the participant through the test. The moderator welcomes the participant, and explains procedure and intention of the test. Then, the moderator guides the participant through test tasks, responds to possible questions, and asks questions in order to investigate on causes for usability problems. At the same time, the moderator observes exactly how the participant solves the tasks, and captures occurring usability problems.
Moderating a usability test is a demanding task. The moderator is responsible for a number of things: s/he makes sure the participant feels comfortable, acts as a time keeper in order not to exceed defined time frames, asks questions, and makes sure that the participant is not distracted or influenced by anything. Especially this last aspect requires practice and experience, as every action might influence the participant and therefore affect test results.