• Salesforce introduced Named Credentials in the Spring’15 release.
• Named credential specifies the URL of a callout endpoint and its required authentication parameters in one definition.
• No need to handle Remote site settings of the Named credential callout URL.
• Named credentials separate the URL from the authentication, making it easier to make changes to both the endpoint URL and authentication if needed.
• It supports Basic Password authentication and OAuth 2.0 authentication protocols.
• You can configured Named credentials to use an org-wide named principal or to use per-user authentication so that users can manage their own credentials.

Apex HTTP Callout Without Named Credential:

String username = 'username';
String password = 'password';
String endpoint = 'https://testapi.com';

Http http = new Http();
HttpRequest req = new HttpRequest();
req.setMethod('POST');
req.setEndpoint(endpoint);

//Add basic authentication header to the callout
Blob headerValue = Blob.valueOf(username + ':' + password); 
String authHeader = 'BASIC ' + EncodingUtil.base64Encode(headerValue); 
req.setHeader('Authorization', authHeader);

HttpResponse response = http.send(req);
System.debug('response-' + response);

Example:
Setup | Quick Find – Search Named Credentials | New Named Credential

Create Named Credential as per your requirement:

Apex HTTP Callout With Named Credential:

Http http = new Http();
HttpRequest req = new HttpRequest();

req.setEndpoint('callout:Sample_API/some_path');
req.setMethod('POST');

HTTPResponse response = http.send(req);
System.debug('response-' + response);