'MetaSel describes a constructor parameter

'MetaSel tells us about a single parameter of a constructor.

'MetaSel fieldname unpackedness sourcestrictness decidedstrictness

parameter	meaning
fieldname	A type level Maybe containing the field name if this is a record field.
unpackedness	The unpackedness of the field.
sourcestrictness	Strictness of this parameter according to the code.
decidedstrictness	Actual strictness decided by GHC, considering compilation flags.

The 'MetaSel is used both to describe a regular parameter and a single field of a record. Both are treated as parameters to a constructor.

data Person = Person { name : Text, age : Int }
data Person = Person          Text        Int

The strictness tells us whether a parameter is strict, meaning values stored in there will be immediately evaluated, or lazy. Haskell allows us to specify which we want when we define the type:

data Humidity = Humidity  Double  -- no annotation (defaults to lazy)
data Humidity = Humidity !Double  -- explicitly strict
data Humidity = Humidity ~Double  -- explicitly lazy

The sourcestrictness option tells us which of the three options above appears in the code. The decidedstrictness option tells us what strictness GHC decided for the parameter, based on the code but also on compiler options, such as the StrictData language extension.