https://www.ofcom.org.uk/siteassets/resources/documents/onli...
It amounts to your basic terms of service. It means that you'll need to moderate your forums, and prove that you have a policy for moderation. (basically what all decent forums do anyway) The crucial thing is that you need to record that you've done it, and reassessed it. and prove "you understand the 17 priority areas"
Its similar for what a trustee of a small charity is supposed to do each year for its due diligence.
Plenty of things in UK law attract "an unlimited fine", but even that doesn't lead to people actually being fined amounts greater than all the money that's ever existed.