Larian's poor tooltips and bad explanations strike again.
Dash lets you use your movement again, as your action. Cunning dash does the same thing - lets you use your movement again, as your bonus action. What this means is that you can move up to three times your normal movement distance, if you do both.
This displays as a 'total of movement' on the circle bar - and rather than refilling your speed, it sets the 'total' figure to double, then triple that amount. This means that when you use dash after fully emptying the bar, it will visually only seem to refill halfway, and if you use cunning dash after that, it will visually only seem to increase by one third. It's a moderately arse-backwards way of managing it, and the tooltips are not as clear or helpful as they could and should be.