Hiya! Mike's quite right, that's how they work...maybe I can also put it another way.
To fill up your tank (or to top off your tank later on):
Step 1: Hook up your Reverse Osmosis system to your sink, garden hose, or whatever other water spigot you decide to attach it to. Directions will tell you how.
Step 2: Get a big bucket (preferably one you've premeasured to know how big, I use 5-gallon buckets), turn on your faucet attached to the RO system, collect the water from the "filtered" end in the bucket and drain the water from the "waste" end. Shut off when bucket is full.
Step 3: Mix the appropriate amount of salt (again, directions) into the full bucket. You should give it a good stir and let it sit for a good time to fully dissolve.
Step 4: Add your newly made saltwater to the tank. Repeat as necessary. If your tank has evaporated water away and you're replacing it, don't add salt--just add your RO freshwater to the tank.
Thus, your RO filter is pretty much your first step. It does make your tapwater pure, but you wanna do that before you add it to the tank. It isn't a filter that runs constantly on your tank or anything like that--it is, in fact, pretty much just a VERY fancy drinking water filter like many people use on their kitchen faucets.
rusty