lazy_load_hooks allows rails to lazily load a lot of components and thus making the app boot faster. Because of this feature now there is no need to require ActiveRecord::Base at boot time purely to apply configuration. Instead a hook is registered that applies configuration once ActiveRecord::Base is loaded. Here ActiveRecord::Base is used as example but this feature can be applied elsewhere too.

Here is an example where on_load method is called to register a hook.

initializer "active_record.initialize_timezone" do
  ActiveSupport.on_load(:active_record) do
    self.time_zone_aware_attributes = true
    self.default_timezone = :utc

When the entirety of activerecord/lib/active_record/base.rb has been evaluated then run_load_hooks is invoked. The very last line of activerecord/lib/active_record/base.rb is:

ActiveSupport.run_load_hooks(:active_record, ActiveRecord::Base)