Q:

What does the arrow in an import statement do?

I've found the the following import in some scala:
import Predef.{println => _, _}

What does the => do?

A:

Generally => in an import allows you to alias an existing name into an alternate name:
import scala.{Int => i32}

This would allow you to use i32 in place of Int
Further, importing _ imports all symbols into the current namespaces.
Now, aliasing a name into _, however does the opposite, i.e. excludes it from the import:
import Predef.{println => _, _}

means

*Import all from Predef except println

