var="hello"

function foo(){
    echo $var
}

foo

Will obviously output "hello", but this works the other way around too:

function foo()  {
    var="hello"
}

foo
echo $var

Will also output "hello"

function foo() {
    local var
    var="hello"
}

foo
echo $var

Will output nothing, as var is a variable local to the function foo, and its value is not visible from outside of it.

var="hello"

function foo(){
    local var="sup?"
    echo "inside function, var=$var"
}

foo
echo "outside function, var=$var"

Will output

inside function, var=sup?
outside function, var=hello