_ is a placeholder for the next lambda argument. So, for example, `x => x.blah()` could be rewritten as `_.blah()`, and `x => blah(x)` could be rewritten as `blah(_)` (or, since we don't need partial application here, simply `blah`).

It also works for multiple arguments: `_ * _` is equivalent to `(x, y) => x * y`, which is really `(x, y) => x.*(y)`.