Global Scope and Functions - Explanation Clarification

Tell us what’s happening:
I am looking for clarification. I have passed the exercise and understand why for the most part. However, the exercise explanation is a little confusing. It states:

" Variables which are used without the var keyword are automatically created in the global scope. This can create unintended consequences elsewhere in your code or when running a function again. You should always declare your variables with var ."

This is confusing because many variables are technically USED without var after being declared. Did it mean that variables that are not DECLARED ( in this case using var) can create unintended consequences?

Link to the challenge:

Yes, basically if variables aren’t declared with keywords such as var, let or const, then it belongs to the global window scope therefore it might cause strange behaviors if you try to use them.

its means using variable with var keyword can be accessed anywhere in the program.sometimes you will forget to name 2 variable with the same name it will make unresolved behaviour in program. you can use let keyword which has a code block scope and const if you want the will the value not be changed further in the program.
example code
constant example
function nodeSimplified(){
const MY_VARIABLE =10;
console.log(MY_VARIABLE); //output 10
MY_VARIABLE =20; //throws type error
let keyword example you can assign another value to a,when the code execute a scope is gone in the block
function nodeSimplified(){
let a =10;
let a =20; //throws syntax error
var can be accessed and used everywhere