Test.getStandardPricebookId() is used to get or fetch Standard Price Book Id in the Apex test class.

Sample Code:

Id pricebookId = Test.getStandardPricebookId();

Sample Apex Test Class:

@isTest
private class ProductTestClass {
    
    static testMethod void testPriceBook() {
        
        Id pricebookId = Test.getStandardPricebookId();
        Assert.isNotNull( pricebookId );
        
    }

}