A muffineer is essentially a salt shaker; it's a small container with a perforated top which is used to shake out powders like sugar, salt, and talc in a controlled way. You may also hear a muffineer referred to as a caster or castor, depending on regional dialect, and of course it is perfectly acceptable to call a muffineer a shaker, since that's what it is.
People have been keeping powders in perforated containers for a very long time, since this storage technique makes a great deal of sense for substances which are used in small volumes. Substances like salt, sugar, and pepper were also once extremely expensive, so storing them in a shaker kept them protected and reduced the risk of waste. Silver salt shakers dating back to the 16th century and even earlier are on display at museums with collections of culinary artifacts, and many of these shakers had very fine, beautiful designs from the hands of master craftsmen.
The word “muffineer” started to be used in the early 1800s to describe a shaker used to hold powdered sugar which could be spread on muffins, scones, and other breakfast goods. 19th century society had a predilection for coming up with a kitchen tool for almost any occasion, and for a time, the muffineer was a distinct and specialized kitchen utensil. Today, vintage muffineers are often sold as “antique salt shakers” or “sugar sprinklers” to collectors, and they are rarely used for their original purpose.
The muffineer concept can also be applied in reverse: there's no reason a modern salt shaker could not be used as a sugar or spice shaker, should one so desire. A shaker could be filled with cinnamon-sugar for sprinkling on toast, for example, or with frequently used spices to make them readily accessible to diners and cooks.
The primary flaw with any sort of shaker is that the powder inside can tend to clump. Muffineers were sometimes historically packed with rice to keep the powder from clumping, a trick which still works, and many companies produced special types of sugar like caster sugar which were specifically processed to avoid clumping so that they could be used in muffineers. Keeping a shaker in a cool, dry place can also help to prevent clumping, and it is a good idea to avoid using muffineers over hot food, as the steam can get inside and cause the contents to clump.