It's an expression and ;; says to ocaml treats this line as expression
3 + 4 ;;
And heres variable declaration or
let x = 3 + 4 ;; (** val x : int = 7 *)
After a new variable is created, the toplevel tells us the name of the variable (x), in addition to its type (int) and value (7).
Also with let you can define a function
let square x = x * x ;; (** val square : int -> int = <fun> *) square (square 2) ;; (** - : int = 16 *)
Function in ocaml is a first class citizen.
;;is required only in repl, not in your source code.