The difference between OCaml's type signatures and type annotations is quite subtle. First of all, signatures are what you put in the mli files, whereas annotations are inline with your code. When you write a function like:
the type of foo resolves to int -> int, because 'a is
serving as a unification variable and not a type parameter. In order to
force the compiler to not constrain the function to the concrete type
int, you need to annotate:
Essentially, this means "for all types 'a", and now we are dealing with a type parameter instead, forcing the compiler to keep the function polymorphic. The mli files work with type parameters by default, so if you are writing type signatures there you might not run into this problem. However, sometimes your function will not typecheck (because it's unclear) until you annotate it with the type ascription, and you'll need to do that before the mli can be checked by the compiler.
There's some more info about this on SO and Jane Street's blog.