The browser's dialog offers three kinds of thing to share. They record differently.
A tab
Pick a tab when what you are explaining is a web page. Only the page is captured. Your other tabs, your bookmarks bar and the rest of your desktop stay out of the picture.
A whole tab is the quickest route: Vistralo switches to that tab and starts the count straight away, without a setup step.
A window
Pick a window when the thing you are showing is not in a browser, such as a design tool or a terminal. The whole window is captured, edges included. After choosing, you see "Set up your recording" and press "Start recording".
The whole screen
Pick the screen when you need to move between applications. Everything visible is recorded, including notifications that pop up. Close anything you would not want a viewer to see.
Changing your mind
On "Set up your recording", "Choose another source" opens the browser's dialog again and "Back" returns to the form.
Recording less than all of it
For any of the three you can record only part of the picture. See Recording part of the screen.