covering that is folded over to protect the contents
"She carefully placed a wax paper folder over the delicate pastry before baking it to keep the crust from drying out."
An organizer that papers are kept in, usually with an index tab, to be stored as a single unit in a filing cabinet.
"She filed the new tax documents into a manila folder with a blue index tab before placing it on the top shelf of the filing cabinet."
In plain English: A folder is a container used to hold and organize papers together.
"Please put all your important documents in this folder before you leave."
Usage: A folder is a physical file holder used to keep documents together for storage or transport. Do not use it to refer to a digital directory on a computer, which is called a directory or simply a folder only by metonymy.
The word folder is formed from the verb fold combined with the suffix -er to indicate an agent or tool. It originally described something that performs the action of folding, which aligns directly with its current meaning as a device for holding papers.