Your water softener should be installed as close to your main stopcock as possible so that your hard water coming in passes through the ion exchange resin beads. These resin beads remove the hardness minerals (calcium and magnesium) from your water permanently making your water soft.
Once the resin bed becomes full of the hardness minerals, the water softener cleans it. This process is known as regeneration. The salt that you put in will create a brine solution and this is used during the regeneration process to clean the resin beads and then it is rinsed away so that there is no salt remaining.
Your water softener will go through this process on a regular basis and you may hear it making a quiet whooshing sound and see some water running out to the drain from the hoses. This may take up to an hour but with more modern units it is often much quicker.
You just need to keep the salt topped up so that it can continue to create the brine solution. Some water softeners will remind you when your salt is running low but if you forget it will not harm your softener, you will just revert to hard water. Top it up again and you will soon get back to lovely soft water.