What does the `and` keyword mean in OCaml?

The and keyword is used either to avoid multiple let (first example, I never use it for this but why not) or for mutually recursive definitions of types, functions, modules…

As you can see in your second example :

let rec debug stack env (r, ty) =
   ...
   | Tunresolved tyl -> o "intersect("; debugl stack env tyl; o ")"
   ...
 
 and debugl stack env x =
   ...
   | [x] -> debug stack env x
   ...

debug calls debugl and vice versa. So the and is allowing that.

[EDIT] It bothered me not to give a proper example so here is one example that you’ll often see :

 let rec is_even x =
   if x = 0 then true else is_odd (x - 1)
 and is_odd x =
   if x = 0 then false else is_even (x - 1)

(You can find this example here)

For mutually recursive types, it’s harder to find a configuration but following this wikipedia page we would define trees and forests as follow

 type 'a tree = Empty | Node of 'a * 'a forest
 and 'a forest = Nil | Cons of 'a tree * 'a forest

As an example, a forest composed of the empty tree, the singleton tree labeled a and a two nodes tree with labels b and c would then be represented as :

 let f1 = Cons (Empty, (* Empty tree *)
             Cons (Node ('a',  (* Singleton tree *)
                         Nil), (* End of the first tree *)
                   Cons (Node ('b', (* Tree composed by 'b'... *)
                               Cons (Node ('c', (* and 'c' *)
                                           Nil), 
                                     Nil)
                           ),
                         Nil (* End ot the second tree *)
                     )
               )
         );;
  

And the size function (counting the number of nodes in the forest) would be :

let rec size_tree = function
  | Empty -> 0
  | Node (_, f) -> 1 + size_forest f
and size_forest = function
  | Nil -> 0
  | Cons (t, f) -> size_tree t + size_forest f

And we get

# size_forest f1;;
- : int = 3