Basic authentication is a simple and widely used method of user authentication for HTTP requests. When a client sends a request to a server that requires authentication, the client includes the username and password (concatenated together and Base64 encoded) in the "Authorization" header of the HTTP request. The server verifies the credentials and grants access if they are valid. Every request sent to the server to a protected endpoint must include these credentials.

Basic authentication is considered insecure for several reasons:

* It transmits user credentials in plain text, making them susceptible to interception and eavesdropping. 
* It relies solely on the server's ability to verify the provided credentials. There is no mechanism for additional security measures like multi-factor authentication or account lockouts after multiple failed login attempts.
* It does not provide a way to manage user sessions securely. The client typically includes the credentials in every request, which creates more opportunities for an attacker to steal these credentials.

These security limitations make basic authentication an insecure choice for authentication or authorization over HTTP.