Salt is a necessary mineral compound. You need it to live. Centuries ago salt was rare enough in some places that it was treated as currency.
When it is humid or damp, salt may clump. So in some places it is common to keep some dry rice in the salt shakers or dispensers. The rice absorbs some of the humidity and can help break apart clumps. It's usually too large to shake out through the dispensers' holes.
If you accidentally add too much salt to a recipe, like a soup or sauce, you can absorb some of it back out by adding a potato.
Salt comes in different forms -- or granularities (Kosher salt, sea salt, salt with iodine). The different forms work better for different purposes. For instance, a large crystal form is useful when using it to melt salt. Other forms dissolve more easily.
Salt is necessary for your body to work correctly. Too much salt can cause problems like high blood pressure.