The ‘Bet view slider’ helps you to see the odds available for the amount you want to bet.
When you log in you will see your default bet view value, which has initially been set to £1 (€1). The bet view defaults to the minimum bet stake (£1/€1) across mobile and desktop devices however, you can toggle to a bet view of £2, £5, £10, £20, £50, £100, £200, £500, or £1000 in your desktop settings. You can change this default value in the ‘Display Options’ . You can also move the slider on any market you look at. Please note that the betting percentage (or overround) will only appear when your bet view slider is set to £1
Let's say you can see this:
You want to bet £40 so you slide the view to £40 to see what odds you can take now:
You can see that £55 is available at odds of 5.2 or better.
Betfair always matches your bet at the best odds available (called ‘Best Execution’), so in this example if you place £40 at 5.2 we'll match £12 of it at odds of 5.4 and the remainder at 5.2.