A coat of plates is like a big, tough shirt that knights used to wear long ago. The shirt was made up of lots of small, flat pieces of metal that were shaped like rectangles and tied together with strong leather cords.
The metal pieces were called plates, and they were nice and shiny which made them look cool. They protected the person wearing the coat from being hurt by swords, arrows, and other weapons.
The knights would wear the coat of plates over their regular clothes to keep them safe during battles. It was heavy and hard to move around in, but it was worth it to protect themselves.
Nowadays, people don't wear coats of plates anymore because we have newer and better ways to protect ourselves. But they are still really interesting to learn about and look at in museums!