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I googled "←" and searched for it on here, unable to find anything. But I found this source code for a chess game. View it here . Example of a code block that has copious usage of this symbol:

 for {
      storedFen ← GameRepo initialFen game
      fen = storedFen orElse (aiVariant match {
        case v@Horde => v.initialFen.some
        case _       => none
      uciMoves ← uciMemo get game
      moveResult ← move(uciMoves.toList, fen, level, aiVariant)
      uciMove ← (UciMove(moveResult.move) toValid s"${game.id} wrong bestmove: $moveResult").future
      result ← game.toChess(uciMove.orig, uciMove.dest, uciMove.promotion).future
      (c, move) = result
      progress = game.update(c, move)
      _ ← (GameRepo save progress) >>- uciMemo.add(game, uciMove.uci)
    } yield PlayResult(progress, move)
The Scala spec says on page 4 that ← (unicode \u2190) is reserved as is its ascii equivalent <- which as others are also pointing out, is an iterator for a for loop.
for(x <- 1 to 5)  println(i)
In scala you can bracket a complex expression and treat it as a single line with value provided by the end of the expression.  What this is doing is creating a large nested for loop where each run through the loop increments the iterator at the end of the loop until, when it recycles, iterates the earlier iterator. 
Here's an example using the scala shell and both syntaxes:
This is how most people write a for loop
Note that the | symbol is a line continuation not typed by me but rather inserted by scala shell
scala> for { 
     | x<-1 to 5
     | y<-2 to 6
     | } println (x,y)
(1,2)
(1,3)
(1,4)
(1,5)
(1,6)
(2,2)
(2,3)
(5,5)
(5,6)
But you can also use that funny arrow unicode symbol and it does the same thing:
scala> for {
     | x ← 1 to 5 
     | y ← 2 to 6 
     | } println (x,y)
(1,2)
(1,3)
(1,4)
(1,5)
(1,6)
(2,2)
(2,3)
(5,5)
(5,6)
You might notice that some of the expressions in that complex for {} block you posted are assignments, not iterations.  That's allowed, and doesn't break the chain of iteration.  Here's a simpler example:
scala> for {
     | x ← 1 to 3
     | y = x*x 
     | z ← 1 to 4
     | } println (x,y,z)
(1,1,1)
(1,1,2)
(1,1,3)
(1,1,4)
(2,4,1)
(2,4,2)
(2,4,3)
(2,4,4)
(3,9,1)
(3,9,2)
(3,9,3)
(3,9,4)
        
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