Why do we need \ in front of Exception in the try catch block?

The backslashes are like a file path or a website address. Each backslash represents another level deep in a nested hierarchy. When you reference a class within the scope of a namespace, the autoloader is going to think that the class you are referencing can be found somewhere in your namespace hierarchy. So if you make reference to Exception within a namespaced class, PHP will (wrongly) think that it is part of the current namespace, and it will result in an error. The problem is, Exception is a built-in PHP class and doesn't have a namespace.

But, if you either use the Exception class at the top of your file, or reference it as \Exception then PHP will know that it is a core PHP class and isn't part of your namespace hierarchy. The prepended backslash is like the equivalent of referencing a file at the root of a directory with ./.

I hope this makes sense. I attempted to explain it in my own words so solidify my understanding, but in case it wasn't clear enough here's a video that explains PHP namespaces: https://symfonycasts.com/screencast/php-namespaces/namespaces