A wearer is the person who wears something — a piece of clothing, a piece of jewellery, glasses, a mask, a uniform. It is made from the verb "wear" plus "-er", the ending that means "the person who does this".

Here is the important thing: almost nobody says this word in normal talking. In real life you say "the person wearing the red hat", or just "she". So when does "wearer" appear? In writing about a THING, when you want to talk about whoever happens to be wearing it, without naming anyone. Shops and labels and instruction sheets do this all the time: "This ring protects the wearer." "The helmet keeps the wearer safe." The sentence is really about the ring or the helmet, and "the wearer" is the empty chair beside it that any person could sit in.

So it turns up most in: product descriptions, safety notes, medical writing (contact lens wearers, hearing aid wearers), fashion writing, and old stories or fairy tales ("the cloak made its wearer invisible"). It feels formal and a little distant, and that distance is the point — it means "anyone at all who puts this on".

You will also meet it stuck to another word: "glasses wearer", "contact lens wearer", "uniform wearer". Those name a type of person by what they habitually wear, which is a bit different from the one-off "the wearer of the crown". Note also that "wearer" usually means clothing and things you put ON your body. A person who wears a beard is not usually called a wearer, though you may see it played with.