A doublet is like a fancy shirt that people wore a long time ago, like in the olden days when kings and queens ruled. It's a piece of clothing that goes on the upper body and has a close fit, sort of like a tight hug.
The doublet usually had sleeves and was made of fancy fabrics like silk or velvet, and it could be decorated with fancy things like buttons or embroidery. People would often wear doublets as part of their fancy outfits when they went to parties or special events.
Some doublets were even padded as a sort of armor to protect the person wearing it in a fight. Overall, the doublet was a type of fancy clothing that made people look important and special.