What's scale bar on CFD charts do/for?

Hiya, am new (to everything) so here’s 1st of my daft questions (I have tried to Google/searched forum, but no dice)-so if you don’t like daft questions, remember my name so you can swerve my posts, 'cause prob gonna be lot of daft questions, ha: what’s the vertical line on CFD charts (adjacent to price column but above graph line)? It has sequence of numbers running up it eg 100, 5000 and it adjusts as you track back in time? I found a post where someone was calling it a scale bar but want to know what it’s for-anyone explain? I really want to move this along but am not going to roll the dice with my minimum wage shekels til I know what all the site-specific buttons do (I m fine researching the generic stuff). Put me out of my misery and I’ll stand on my front step tonight and clap you.