The numbers are irrelevant. It's just an arbitary scale.

If you were experiencing a pain that's so mild it's basically just a feeling of discomfort, you'd be able to describe that, right? And you it you were experiencing a pain so intense that it has you literally screaming? You'd be able to describe the difference between them, right? Well there's literally a conversion chart that translates those feelings into numbers.

You don't have to conceptualise anything — it's literally printed on a paper for you when they ask you the question. As long as you can experience pain and either read a description of pain severity, or describe your pain severity to someone who can read, you can use a numerical pain scale.