This code inform the client that the request has been received by the server.The response will be send to the client after after the request has been fully received and acted upon.

Its also notified the client that the server has not rejected your request and working on that.

If the request contains an Expect header field that includes a 100-continue expectation, The 100 response notify that the serverwishes to receive the request payload body, The client continue continue sending the request and discard the 100 response.

if not defined Expect header field into the request then the client can simply discard this interim response.