I am far more likely to over-season food than under-season it. I love salt, probably more than most, and every once in a while the salt shaker gets away from me. Unfortunately, once that sodium chloride hits your food, it’s very difficult to remove (especially if you’re making a sauce, soup, or any other mostly-liquid meal).
You may have heard that a piece of raw potato can “soak up” the excess salt, but that is just not true. Potatoes are incapable of performing reverse osmosis. They do absorb water, including salty water, but all that does is reduce your total volume of salty water — the ratio of salt to water will remain virtually unchanged. (If you want to remove some of the salty water, just pour it out.)
To fix over-salted food, you need to skew the ratio of salt-to-food back towards the food. The easiest way to do this is to add more food to the pot, whether that would be more of the same food or something entirely different. For an over-seasoned tomato sauce, try adding a few more canned tomatoes (or wine!); for a soup, add more broth or water; for a stew, try adding more vegetables (yes, including potatoes).
Another strategy is to make flavours fight for your tongue’s attention by adding a competing ingredient. Sugar, acid, and fat are all particularly good at offsetting salinity. A few teaspoons of table sugar or a drizzle of honey can balance out the extra salt, as can a squeeze (or three) of lemon or splash of vinegar. Rich, heavy cream (or some other full-fat dairy) not only dilutes, but mutes, making the salt harder to taste.
If you are working with a liquid-heavy meal, you can drain some of the liquid away and replace it. If you’re working with a large chunk of over-seasoned meat, try wiping some of the seasoning off. If the salt has permeated the meat, chop it up and use it as an accent (like you would bacon) rather than making it the focus of your meal. If you’re making a big pot of of little things (like beans, rice, or some other grain), you can add more of those things, chuck it all into an under-seasoned soup or stew, toss it all with an unsalted sauce, or try a combination of these tactics. Just don’t expect a single raw potato to fix your salty problem. Potatoes are quite magical, but they just aren’t capable of selectively removing sodium chloride ions from a broth, stock, or sauce.