A Method is what we call a workflow (or subset of a workflow) in FireCloud. A workflow is typically composed of several tasks meant to be performed in a certain order, with some amount of interdependency between them. For example, the outputs of one task may be used as inputs to the next task, and so on until the full set of tasks has been run.
There are many ways to define a workflow, i.e. to describe the series of tasks involved, in a way that computers can understand. The workflow execution system embedded in FireCloud, which is called Cromwell, uses a scripting language called WDL (Workflow Description Language, pronounced "widdle"), which you can learn more about here. WDL allows you to include in your workflows any executable program that you would normally run on the command line. It uses references to Docker containers to make the executable programs available to the workflow execution system.
You can find WDL workflows that are ready to run in the Method Repository, or you can import your own (see the Tutorials section for more details).