What is a role in ansible and what is the directory structure

Roles

Roles let you automatically load related vars, files, tasks, handlers, and other Ansible artifacts based on a known file structure. After you group your content in roles, you can easily reuse them and share them with other users

Role directory structure

An Ansible role has a defined directory structure with eight main standard directories. You must include at least one of these directories in each role. You can omit any directories the role does not use. For example:

# playbooks
site.yml
webservers.yml
fooservers.yml
roles/
    common/
        tasks/
        handlers/
        library/
        files/
        templates/
        vars/
        defaults/
        meta/
    webservers/
        tasks/
        defaults/
        meta/

By default Ansible will look in each directory within a role for a main.yml file for relevant content (also main.yaml and main):

tasks/main.yml - the main list of tasks that the role executes.
handlers/main.yml - handlers, which may be used within or outside this role.
library/my_module.py - modules, which may be used within this role (see Embedding modules and plugins in roles for more information).
defaults/main.yml - default variables for the role (see Using Variables for more information). These variables have the lowest priority of any variables available, and can be easily overridden by any other variable, including inventory variables.
vars/main.yml - other variables for the role (see Using Variables for more information).
files/main.yml - files that the role deploys.
templates/main.yml - templates that the role deploys.
meta/main.yml - metadata for the role, including role dependencies.