What does the underscore(_) do in for loop Golang?

I am just getting started learning the Golang language!
In for loop, I saw sometimes adding an underscore or without underscore.

Whatever add _ or not, I got the same result.

package main

import (
	"fmt"
)

func main() {
	doSomething()
	sum := addValues(5, 8)
	fmt.Println("The sum is", sum)

	multiSum, multiCount := addAllValues(4, 7, 9)
	fmt.Println("multisum", multiSum)
	fmt.Println("multiCount", multiCount)
}

func doSomething() {
	fmt.Println("Doing Something")
}

func addValues(value1 int, value2 int) int {
	return value1 + value2
}

func addAllValues(values ...int) (int, int) {
	total := 0
	for _, v := range values {
		total += v
	}
	return total, len(values)
}

func addAllValues(values ...int) (int, int) {
	total := 0
	for v := range values {
		total += v
	}
	return total, len(values)
}

All I know is I don't care about the index. Is that all? or there is something more what I have to know??

I really appreciate your help!

For range over slices:

1. In for v := range values { the v is the index of the element in the slice.
2. In for _, v := range values { the v is the actual element value.
3. In for i, v := range values { the i is the index and the v is the element.
4. In for i, _ := range values { the i is the index of the element in the slice.

You can run this playground example to see the differences.

Range expression                          1st value          2nd value

array or slice  a  [n]E, *[n]E, or []E    index    i  int    a[i]       E
string          s  string type            index    i  int    see below  rune
map             m  map[K]V                key      k  K      m[k]       V
channel         c  chan E, <-chan E       element  e  E

For more details see the spec.

If you don't want to use the variable that iterates in the loop, you can use _ to simply let Go ignore it:

mySlice := [int]{1,3,4,59,5}
for _,x := range mySlice {
    fmt.Println(x)
}

By placing underscore you are telling the compiler this:
Ok, I'm aware that this function is returning something but I don't care! For example:

package main

import "fmt"

func main() {
	mul1, add1 := test_function(2, 3)
	fmt.Println(mul1, add1)

	mul2, _ := test_function(4, 5)
	fmt.Println(mul2)

	_, add3 := test_function(7, 8)
	fmt.Println(add3)
}

func test_function(a int, b int) (mul int, add int) {
	return a * b, a + b
}

just to add to the amazing answer above:
I think one of the main benefits is to maintain readability in your program: if you replace the blank identifier with a variable then you have to use it or your program will not compile.
also this decrease memory allocation be neglecting one of the returned parameters...