What are a "named object", a "named list" and a list of objects in YAML?

A named object is a key whose value is an object; a named list is a key whose value is a list; a list of objects is a list whose items are objects (a dash starts each object, its other keys line up below it without dashes).

user:            # named object (key-value pair with an object)
  name: "Bob"
  age: 30

fruits:          # named list (key-value pair with a list)
  - apple
  - banana

users:           # named list whose items are objects
  - name: Alice
    role: admin
  - name: Bob
    role: user

Rules worth remembering:

• A key-value pair can hold anything as its value: a scalar, an object or a list.
• Keys inside an object have no dashes. Only list items do.
• An object always consists of key-value pairs. There is no object "directly with" a list; a list can only appear as the value of a key (or as an item of another list).
• In users, the list has three items (dashes) and each item is an object with two keys. The dash belongs to the first key of each object; role lines up under name with no dash because it is in the same object.

This last shape, a named list of objects, is exactly what tasks: in a play is.

Go deeper:

• YAML Syntax (official docs) — nested lists and dictionaries as Ansible uses them.