A journey is the set of instructions that mabl must execute in order to verify that your application is functioning as expected. Instructions could include visiting a particular URL, clicking on a given field in a form, entering text, clicking on a button, verifying that a field has a certain value, verifying that an object loads, and more.
A plan specifies how mabl should test a journey. It tells mabl which application to test, which credentials to use (if applicable), which journeys to test, which environments to test against, and when the tests should be executed.