Why remote site settings is necessary?

What is remote site settings:

Before any Apex callout to an external site, that site must be registered in the Remote Site Settings page. Otherwise, a callout can fail. Because of the security, Salesforce prevents calls to an unauthorized network address.

Please follow the below Steps :

Go to Setup ► Security Controls ► Remote Site Settings

For example, we are using Amazon S3 storage server and Multipart as bucket name. 

⦁ Remote Site Name: Multipart (Bucket Name as specified in your server) [ Multipart is used as example]

⦁ Remote Site URL: https://multipart.s3.amazonaws.com

⦁ Check the Active box

⦁ Save the settings

Note: In the above example, Remote Site name can choose any name as per their requirement.