If you make a change to your named.conf that you want named to start using then use rndc reconfig.
rndc reconfig will also load any new zones that you've added (and remove any that you no longer have defined), but it won't detect any changes that you've made to zones that are already loaded.
If you're making changes to zone files (remember to increment your serial number in the SOA record!) then you'll need to use rndc reload, but on a server with a lot of zones or several large zones, you can minimize the impact on its responsiveness to queries by reloading only the zones that you have updated using rndc reload <zone name> for each.
If you are using multiple views you will need to tell rndc which view(s) to reload in by specifying the class and the view name as arguments to rndc, e.g. rndc reload <zone name> in <view name>
rndc reload of all zones may not be your best option, even though it is the easiest
Although this has been improved in BIND 9.8.2 and newer, a full rndc reload on a busy server with many authoritative zones can incur significant overhead and affect server performance while it is running.