/.well-known/ is defined in the RFC 5785. The .well-known folder is essentially a directory for storing site-wide metadata files. In order to prevent naming conflicts the RFC specifies a register of well-known URI’s. It also has the added bonus of not cluttering up the root directory with metadata files.
If you are familiar with cPanel/WHM control panel you might see the file named .well-known in your cPanel account file directories. When you see this named one don’t get panic this folder automatically created by cPanel in order to validate your domain for AutoSSL purposes. AutoSSL is an added feature of cPanel/WHM which offer you free SSL certificate for your domains, its also known as self-signed SSL certificate. The folder .well-known created while the time of the domain validation process as a part of AutoSSL installation.