Marks the event safe for network use. Aka, allows you to trigger the eventName event on the client, from a server side script. If you do not provide a callback function use AddEventHandler to listen for the event after registering it.
RegisterNetEvent(string eventName, function callback)
RegisterNetEvent('eventName', function(...)
print('Event fired')
end)
or
RegisterNetEvent('eventName')
AddEventHandler('eventName', function(...)
print('Event fired')
end)