'MetaData describes a type

The 'MetaData parameters provide information about the original type.

'MetaData typename modulename packagename isnewtype

parameter	meaning
typename	The name of the type. For data Unicorn this is Unicorn
modulename	The module in which this type is defined, e.g. “My.Module”
packagename	The package in which this type is defined, e.g. “my-package”
isnewtype	'True if defined using newtype, 'False if defined using data

The prime in 'MetaData isn’t a typo and really part of the name. 'MetaData gets this prime because it is defined in a peculiar way. We usually define types using the keywords type, newtype, or data, but there’s another way: If we enable the DataKinds extension in a module GHC will create a type for each constructor in the module, with the same name as that constructor. That’s why in the documentation for 'MetaData we see it defined as a constructor of the Meta type.

DataKinds will give us a type and a constructor with the same name. When we put a prime in front of the name we tell GHC we mean the type, not the constructor. In most cases we can also omit the prime, and GHC will figure out whether we mean the constructor on the type based on where the name appears.

The generated types DataKinds produces don’t have any values so their only real use is in phantom types, which is exactly how 'MetaData' is used!

If you’re interested in learning more about DataKinds check out this blog post on kinds.