modules_enabled is the list of modules Prosody will load when started. An example looks like this:

   Host "*"
        modules_enabled = {
                                "saslauth"; -- Authentication for clients and servers. Recommended if you want to log in.
                                "legacyauth"; -- Legacy authentication. Only used by some old clients and bots.
                                "roster"; -- Allow users to have a roster. Recommended ;)
                                "register"; -- Allow users to register on this server using a client
                                "tls"; -- Add support for secure TLS on c2s/s2s connections
                                "vcard"; -- Allow users to set vCards
                                "private"; -- Private XML storage (for room bookmarks, etc.)
                                "version"; -- Replies to server version requests
                                "dialback"; -- s2s dialback support
                                "disco"; -- Service discovery
                                "ping"; -- XMPP Ping
                                "time"; -- Let others know the time here
                                "uptime"; -- Uptime reporting
                                "pep"; -- PEP support (tune, mood, actvity, ...)
                          };

It is a comma (or semi-colon) separated list. The name of a module corresponds to its filename (mod_X.lua in your Plugins Directory).