New to Splunk> and trying to wrap my head around things... I have a host (it's a Linux Firewall/IDS) that is transmitting all syslog data to my Splunk server. I'm really only concerned about the iptables drops and snort alerts.
If your logs are getting indexed and the fields are properly parsed with that config then you have done good so far.
If you want to now drop all but the iptables, drops, and alerts, then you will need to configure props.conf and transforms.conf to save only those events you want to keep. If you decide to drop data at index time then understand that the data will be lost, and will need to be reindexed if you find you need it in Splunk assuming it is also getting stored elsewhere.