This "package" is a dependency for your package(s). It allows your users to enable/disable a key binding by simply changing your package's settings. And all it requires you to do is to add:

"context": [
    { "key": "package_setting.SettingsFileName.setting_to_enable_this_key_binding" }
]

That's it!

To make things clearer, here's when it's decomposed:

1. package_setting.: this prefix is needed and the exact same in every cases.
2. SettingsFileName: this part might be a bit more confusing. It the name of the setting file without the extension in which you write custom settings for the package. It usually is the same as the package name. It also usually have the extension .sublime-settings.
3. setting_to_enable_this_key_binding: simply the setting's name. It can be a bit more than that though, keep reading to get more 😉

A YourPackage.sublime-settings file:

{
    "enabled_shortcut_my_super_command": true
}

[
    {
        "keys": ["ctrl+l"],
        "command": "my_super_command",
        "context": [
            { "key": "package_setting.SettingsFileName.enabled_shortcut_my_super_command" }
        ]
    }
]

And this shortcut will work. But if you set YourPackage.sublime-settings's content to:

{
    "enabled_shortcut_my_super_command": false
}

You guessed it: this key binding wouldn't work any more. Pretty handy, right?

In the overview's example, the operator: equal and operand: true is implicit, but you can of course change it:

[
    {
        "keys": ["ctrl+l"],
        "command": "my_super_command",
        "context": [
            { "key": "package_setting.SettingsFileName.enabled_shortcut_my_super_command",
              "operand": "not_equal",
              "operator": "a string for example!"}
        ]
    }
]

The key is quiet clever too. Have a look:

In Hello.sublime-settings:

{
    "my_setting": {
        "my_key": ["my", "list", {
            "and": "it goes on and",
            "on": true
        }]
    }
}

And your key binding:

[
    {
        "keys": ["ctrl+l"],
        "command": "my_super_command",
        "context": [
            { "key": "package_setting.SettingsFileName.my_setting.my_key.2.on"}
        ]
    }
]

And it works!

If you want to use this feature in your package, just add it as a dependency,[]. So, for example

In your dependencies.json

{
    "*": {
        "*": [
            "PackageSettingContext"
        ]
    }
}

What you can do is actually set this as a dependency to your User package. Create the file Packages/User/dependencies.json, and set the same content as above