'-' hyphen is used to specify list items. If you know Python, here's an example that might help you understand this better.
my_list = ['foo', 'bar']
In Ansible you will specify these list items with hyphens:
Inside a playbook, you may have a list of plays and inside each play, you may have a list of tasks. Since tasks are nothing but lists, each task item is started with a hyphen like this: