请教SCALA函数的问题

Question

object ScalaFunction {
def main(args: Array[String]): Unit = {
def fun1(): String = {
"zhangsan"
}
val a = fun1
val b = fun1 _
val c : ()=>Unit = fun1
println(a)
println(b)
}
}

val b = fun1 _ 和 val c : ()=>Unit = fun1 是啥意思啊 没看懂