Lying still while gongs, bowls and voice are played around you, usually for an hour or two. Genuinely relaxing and a reasonable first step for someone who has never done anything like this. The physiological claims made for specific frequencies are not supported by evidence, and a good facilitator will not make them.
The honest description is that sustained overlapping tone gives attention something to rest on and makes lying still for an hour easy, and that many people drift into hypnagogia, the state between waking and sleep, where imagery arrives on its own. That is a real and pleasant effect and it needs no further explanation.
What gets stacked on top of it is not real. Claims about specific hertz values healing specific organs, DNA repair, or cellular frequency are marketing. They are also a useful filter: a facilitator making them is telling you how they think about evidence.
There is a separate and much older thing under the same heading. Nada yoga treats sound, including internal sound, as the object of meditation rather than as an experience delivered to you. It is a practice you learn, not a session you attend.
A representative shape drawn from how these are typically run. Use it as something to ask against, not as any one centre's timetable.