List moderators are nominated by list managers, and assist with the approval or rejection of held postings. Moderators cannot change any other aspect of a list.
List moderators see a "Moderate" link in the List Home page for each list that they moderate. This leads to https://lists.cam.ac.uk/sympa/modindex/list-name (replace list-name with the name of your list).
When an email is held for moderation, the list moderators will (normally) receive an email.
This email contains a link to the same page. It also contains instructions to approve or reject the message by sending emails to sympa@lists.cam.ac.uk of the form:
DISTRIBUTE list-name 666249e7015006d46d0e406b2cf61339
REJECT list-name 666249e7015006d46d0e406b2cf61339
(The text on the right hand side is a one-time authentication token to authorise the action).
The email message also contains mailto: links which will automatically perform this action in many environments.