What is @testSetup in Salesforce?

Methods defined with the @testSetup annotation are used for creating common test records that are available for all test methods in the class.

@testSetup: You can write a method in the test class, with @TestSetUp annotation applied, and create all your common test data in this method. Data created in this method doesn’t need to be created again and again, and it is by default available for all test methods.