Functions in Haskell are one of its fundamental constructs. Some familiar functions like addition or multiplication are binary, taking two arguments and returning a single value. Formally, functions are binary relations between two sets, such that each member of the first set maps exactly to one member of the second set. 1 + 1 will always return 2, at least in decimal.
In Haskell, functions are largely defined not only by their definitions,
f x = x
but also by their declarations, which contain the types of their inputs and outputs.
f :: Integer -> Integer
bounds x to the set of Integers, which maps directly to the set of Integers. (1 == 1, after all)
Functions can be thought of abstractly as black boxes, which take input and return output. This is the basis for the lambda calculus, another heavy influencer of Haskell.