Bearer tokens, such as JSON Web Tokens (JWT), are a means to ensure that the party making a request to a server is the one it claims to be. When a user authenticates, they receive a token that they include in the header of their HTTP requests. The server validates this token before proceeding with the request, thus securing the application by authenticating the client. Other options listed do not perform the role of bearer tokens because SSL/TLS is used for secure transmission, one-time passwords are typically used for multi-factor authentication, and Amazon Resource Names (ARNs) are used to identify AWS resources, not for securing application transactions between clients and servers.