Sometimes when you use underscores as placeholders for parameters, the compiler might not have enough information to infer missing parameter types. For example, suppose you write _ + _ by itself:

scala> val f = _ + _
<console>:4: error: missing parameter type for expanded
function ((x$1, x$2) => x$1.$plus(x$2))
val f = _ + _
ˆ

In such cases, you can specify the types using a colon, like this:

scala> val f = (_: Int) + (_: Int)
f: (Int, Int) => Int = <function2>
scala> f(5, 10)
res9: Int = 15

Note that _ + _ expands into a literal for a function that takes two parameters. Multiple underscores mean multiple parameters, not reuse of a single parameter repeatedly. The first underscore represents the first parameter, the second underscore the second parameter, the third underscore the third parameter, and so on.
Read more: Placeholder syntax