Http response or Status code that meaning response from the server when the client requests to the server and sends back the response to the client from the server. This is the server-side response code that meaning those words.

Http Status Code

• 200 means everything is ok on the server-side and server-side is returning the data correctly
• 201 means that when you try to create an object on the server-side it has been successfully created.
• 301 means that whatever you are requesting has moved permanently to a new location and the URL of the new location of that resource will be returned to your client-side.
• 400 and 500 typically indicate that there is some problem with the server-side.
• 404 means request for something that doesn't exist on the server-side.
• 500 means that the server is just giving up, it is unable to process your request and then sends back and  internal server error.
• 505 Http version not supported

All those are essential "status code" that you have to know it and apply on your project and take it easy learning