I don't understand what a YAML tag is

I get it on some level, but I have yet to see an example that didn't bring up more questions than answers.

http://rhnh.net/2011/01/31/yaml-tutorial

# Set.new([1,2]).to_yaml
--- !ruby/object:Set 
hash: 
  1: true
  2: true

I get that we're declaring a Set tag. I don't get what the subsequent hash mapping has to do with it. Are we declaring a schema? Can someone show me an example with multiple tag declarations?

I've read through the spec: http://yaml.org/spec/1.2/spec.html#id2761292

%TAG ! tag:clarkevans.com,2002:

Is this declaring a schema? Is there something else a parser has to do in order to successfully parse the file? A schema file of some type?

http://www.yaml.org/refcard.html

Tag property: # Usually unspecified.
    none    : Unspecified tag (automatically resolved by application).
    '!'     : Non-specific tag (by default, "!!map"/"!!seq"/"!!str").
    '!foo'  : Primary (by convention, means a local "!foo" tag).
    '!!foo' : Secondary (by convention, means "tag:yaml.org,2002:foo").
    '!h!foo': Requires "%TAG !h! <prefix>" (and then means "<prefix>foo").
    '!<foo>': Verbatim tag (always means "foo").

Why is it relevant to have a primary and secondary tag, and why does a secondary tag refer to a URI? What problem is being solved by having these?

I seem to see a lot of "what they are", and no "why are they there", or "what are they used for".

I don't know a lot about YAML but I'll give it a shot:

Tags are used to denote types. A tag is declared using ! as you have seen from the "refcard" there. The %TAG directive is kind of like declaring a shortcut to a tag.

I'll demonstrate with PyYaml. PyYaml can parse the secondary tag of !!python/object: as an actual python object. The double exclamation mark is a substitution in itself, short for !tag:yaml.org,2002:, which turns the whole expression into !tag:yaml.org,2002:python/object:. This expression is a little unwieldy to be typing out every time we want to create an object, so we give it an alias using the %TAG directive:

%TAG !py! tag:yaml.org,2002:python/object:            # declares the tag alias
---
- !py!__main__.MyClass                                # creates an instance of MyClass

- !!python/object:__main__.MyClass                    # equivalent with no alias

- !<tag:yaml.org,2002:python/object:__main__.MyClass> # equivalent using primary tag

Nodes are parsed by their default type if you have no tag annotations. The following are equivalent:

- 1: Alex
- !!int "1": !!str "Alex"

Here is a complete Python program using PyYaml demonstrating tag usage:

import yaml

class Entity:
    def __init__(self, idNum, components):
        self.id = idNum
        self.components = components
    def __repr__(self):
         return "%s(id=%r, components=%r)" % (
             self.__class__.__name__, self.id, self.components)

class Component:
    def __init__(self, name):
        self.name = name
    def __repr__(self):
        return "%s(name=%r)" % (
            self.__class__.__name__, self.name)

text = """
%TAG !py! tag:yaml.org,2002:python/object:__main__.
---
- !py!Component &transform
  name: Transform

- !!python/object:__main__.Component &render
  name: Render

- !<tag:yaml.org,2002:python/object:__main__.Entity>
  id: 123
  components: [*transform, *render]

- !<tag:yaml.org,2002:int> "3"
"""

result = yaml.load(text)

More information is available in the spec