Unit tests are methods that verify whether a particular piece of code is working properly. Unit tests can be run synchronously or asynchronously, and can be used to test Apex classes, triggers, and Visualforce controllers.
Unit tests can also use system methods to set or override certain system properties, such as the current user, the current time, or the created date of a record. These methods are useful for testing different scenarios or behaviors that depend on these properties. For example, a unit test can use the System.runAs() method to execute a block of code as a different user, or the System.Test.setCreatedDate() method to set the created date of an sObject to a past or future date. See Testing Apex1 for more details. References: 1: Testing Apex | Apex Developer Guide | Salesforce Developers(https://stackoverflow.com/questions/767567/what-to-test-in-unit-testing-a-method-or-a-scenario)