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.
From the linked document above: "You need to keep a record of each illegal content risk assessment you carry out", "service providers may fully review their risk assessment (for example, as a matter of course every year)"
And links to a guidance document on reviewing the risk assessment[1] which says: "As a minimum, we consider that service providers should undertake a compliance review once a year".
[1] https://www.ofcom.org.uk/siteassets/resources/documents/onli...