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.