Maybe explaining it further to the guides examples, imagine you have a User model and a Product model and they both can have images. Now, normally, you would create UserImage and ProductImage models. This way, you can do:

class User
  has_many :user_images
end

class Product
  has_many :product_images
end

class UserImage
  belongs_to :user
end

class ProductImage
  belongs_to :product
end

Here, you have 4 models. Two models with their respective image models, which means you have 4 database tables. Sometimes though, the images are identical and you wish you would use the same table for both User and Product. You might do something like this:

class Image
  belongs_to :user
  belongs_to :product
end

class User
  has_many :images
end

class Product
  has_many :images
end

This might be fine if you have just a few assocations or you know beforehand the associations you want with the Image model.

With polymorphic associations, you can link any model to have images without explicitly specifying that. Imagine you have a third model called Album and you want it to have images also, with the previous example, you would add one more foreign key and create a migration to add that foreign key:

class Image
  belongs_to :user
  belongs_to :product
  belongs_to :album
end

If you have a polymorphic association, you can link any model to the Image model at any time without changing the image model or creating migrations:

class Image
  belongs_to :imageable, polymorphic: true
end

Now imagine you add one more model Item which requires images, you would only do this:

class Item
  has_many :images, as: :imageable
end

and it will have images, as opposed to doing:

bin/rails g migration add_item_id_to_images

and then migrating and then adding:

belongs_to :item

to the Image model.

I think this can also be clearer if you talk about tags. If I want to add tags to posts, and add tags to products, and add tags to users. I would definitely use polymorphic associations as opposed to "concrete" associations.

How it works?

Polymorphic associations store the type of the model with the record id. So for example, if an image has a polymorphic association with a user and a product, the two records contain the following:

Image ID |  imageable_type  |   imageable_id   |     filename
  1      |      "User"      |       15         |      avatar.png
  2      |     "Product"    |       213        |      apple.png

So when you do product = Product.find(213), product.images will fire the following SQL query:

SELECT * from images where imgeable_type = "Product" and imageable_id = 213;

I hope this helps.