What is a "conditonal" GET method?

Govind Seshadri

If a GET request method contains any additional headers like If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match or If-Range, then the request is known as a "conditional" GET. In this case, the entity will be transferred to the client only if the conditions specified in the header fields are satisfied. Conditional GETs can be used especially by caching browsers to reduce unnecessary network traffic by ensuring that data already present within the client is not transferred again.