what does ${} mean bash code example

Example: $@ bash

file:test.sh
#! /bin/sh
echo '$#' $#
echo '$@' $@
echo '$?' $?

*If you run the above script as*

> ./test.sh 1 2 3

You get output:
$#  3
$@  1 2 3
$?  0

*You passed 3 parameters to your script.*

$# = number of arguments. Answer is 3
$@ = what parameters were passed. Answer is 1 2 3
$? = was last command successful. Answer is 0 which means 'yes'