Can someone tell me what this blue icon is on the left side of my Artboard please
That is where the viewable bottom of the artboard is. Any content below that line is not initially visible and the user will need to scroll to view it. The line is adjustable. It's initial value is what initial size of the artboard when it was created.
Thanks Chris for the explanantion. I just couldnt find an answer, you have fully cleared up my question now.
To add one more thing to what @Chris W. Griffith said, keep in mind that the line is the bottom of the screen so if you're designing a webpage you won't be able to fill that entire screen with your page content. That's because the web browser UI will take up some space (plus the phone's status bar, etc). So for websites count on seeing less than that before having to scroll. If you're designing an app, be sure to include the phone's status bar, etc for a true idea of how much content will fit before they have to scroll.
Thank you to all for explaining what this is