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"

By default, every variable in bash is global to every function, script and even the outside shell if you are declaring your variables inside a script.

If you want your variable to be local to a function, you can use local to have that variable a new variable that is independent to the global scope and whose value will only be accessible inside that function.

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