Hi Muhammad! As explained above, a block is a portion of code enclosed by curly braces: {}. An if statement, for example, would look something like this:

let y = 2;
if (y = 2) {
    let x = 5;
}

In the above example, x cannot be accessed outside that block. If we were to run this:

let y = 2;
if (y = 2) {
    let x = 5;
} 
console.log(x);

The console would return a reference error; because outside that block, x behaves as if it has not been declared at all. Variables declared within a block using let or const cannot be referenced outside that block, and will essentially not be 'recognized' outside of that block.