It's just an identifier like any other defined in types.ts: https://github.com/MathisBullinger/froebel/blob/main/types.t...

It's defined as

  type λ<TA extends any[] = any[], TR = any> = (...args: TA) => TR;

meaning λ is just any function and λ<[number], string> would be the type of a function that takes a number as its only argument and returns a string.