In other words, salt is sprinkled on the ring to purify it.
By the way, in the case of sumo, there are many things that take place between the time the player steps into the ring and the time he or she returns home. Not only the salt, but each of the other actions has a meaning.
For example, they rinse their mouths out with a ladle of water. This is called "chikaramizu" and is meant to purify the body.
Shiko is performed in the ring. Shiko-fumu means to crush the bad guys underground.
There is also the action of crouching down, clapping one's hands, and then spreading them out. This is to show the opponent that you do not have any weapons in your hands.
Sumo is an old Japanese sport with a long history. It seems that even the smallest gesture has its own meaning. There are many more secrets to Sumo than those I have mentioned here. If you watch sumo with these facts in mind, it will become even more interesting.