Testing is the key to successful long-term development and is a critical component of the development process. Unit tests validate that your application works as expected, that there are no unexpected behaviors.
Apex provides a testing framework that allows you to write unit tests, run your tests, check test results, and have code coverage results.
One of the requirements from Salesforce to deploy Apex code to the production environment or upload package to the Salesforce AppExchange related with Ape[ code coverage. Unit tests must cover at least 75% of your Apex code, and those tests must pass.
Code coverage indicates how many executable lines of code in your classes and triggers have been exercised by test methods.
Code coverage serves as one indication of test effectiveness but doesn’t guarantee test effectiveness. The quality of the tests also matters, but you can use code coverage as a tool to assess whether you need to add more tests. While you need to meet minimum code coverage requirements for deploying or packaging your Apex code, code coverage shouldn’t be the only goal of your tests. Tests should assert your app’s behavior and ensure the quality of your code.