Testing in a CI/CD pipeline involves various types of tests at different stages including unit tests, integration tests, user acceptance tests, load tests, and disaster recovery tests. The document outlines different types of tests like unit testing, functional testing, integration testing, user interface testing, A/B testing, penetration testing, performance testing, system testing, smoke testing, and disaster recovery testing that are commonly used at stages like development, QA, staging, and production. Mabl uses machine learning to help simplify and improve CI/CD testing pipelines.