`let <var> = <expr> in <expr>` is an expression. Top-level bindings are just `let <var> = <expr>`. That’s pretty much all there is to it.

    let fac =
      let rec fac' acc = function
        | 0 -> acc
        | n -> fac' (n * acc) (n - 1)
      in
      fac' 1

    let seven =
      let four = 4 and three = 3 in
      four + three

https://ideone.com/HpTrI4

The 'in' keyword is purely syntax, like semicolons/newlines or braces in your language of choice.

Never used OCaml but it seems like a way to chain together expressions using the same variable name? Seems odd but I could see myself using it