A Brief Look at Untyped Lambda Calculus
When talking about Haskell, the term “lambda calculus” often crops up. It’s a theoretical framework used to define the meaning of computation in many functional languages, such as Haskell, Agda, Idris, etc. Understanding lambda calculus can be very helpful when talking about programs written in these languages.
Source: A Brief Look at Untyped Lambda Calculus, an article by Nikolay Yakimov.