Hyphen - is used to specify list items
 my_list = ['foo', 'bar']
In Ansible you will specify this list items with hyphen:
my_list:
  - foo
  - bar
Inside a playbook you have a list of plays and inside each play you have a list of tasks. Since tasks are lists, each task item is started with a hyphen like this:
tasks:
  - task_1
  - task_2