Unless you have run across an application where you need one, you have probably never heard of a C spanner. This is usually just a flat piece of metal shaped somewhat like a spoon seen from the side. There is a long flat place for you to grab onto it, and a C-shaped curve built into it.
This curve is customized to fit a specific radius so that it will fit tightly against it. There is usually just one nub sticking out of the C shape, which fits into a little hole on the item the spanner was designed to tighten or loosen up.
This is usually some sort of lock or retaining ring and is often in very tight quarters where an ordinary nut and wrench would not work due to clearance issues. Don’t ever throw one out unless you know what it was used for, since there aren’t any good substitutes for one if it turns out you need it after all.