CORS: origin is not allowed access [duplicate]

This question already has an answer here:

• “No 'Access-Control-Allow-Origin' header is present on the requested resource” 13 answers

I got this error when doing an ajax call to an endpoint

Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:8888' is therefore not allowed access.

but why in POSTMAN I tried with the same params and endpoint there's no problem getting the data?