I am learning Lambda Calculus from the book by Hindley and Seldin . They start the formal postulation of lambda calculus as follow :
(a) all variables and atomic constants are λ-terms (called atoms);
(b) if M and N are any λ-terms, then (MN) is a λ-term (called an application);
(c) if M is any λ-term and x is any variable, then (λx.M) is aλ-term (called an abstraction).
In the second postulate $MN$ has been termed as a $\lambda$ term . How to define $MN$ , what does it mean ? Is it an operation? Is there any scope of defining operations in lambda calculus which are associative and/or distributive ?