`|` is read as "or". `::` is syntax for what Lisp and Scheme call `cons`, which places an element onto either a list or the empty list, `[]`. The syntax `'a` is a type variable, which means that `'a` can be any type but it must all be the same type everywhere `'a` appears.

So this type definition means that `'a list` is a list of elements of type `'a`, and it's defined by being something of type `'a` consed onto either another list of type `'a` OR the empty list, represented by `[]`.