Boosting is a core mechanic of Untold - it is the ability of your character to enhance the effect of any powers tied to that Aspect. How do you know? Look to the Boost test area on any Race, Minion, or Power Card. The picture below demonstrates how boosting works:
Since this Aspect gives you 3 Body Boost, that means you can add 3 to the Magnitude (MAG) of the Power.
I.E. "For every 2 Boost Add 3 to MAG", and we have 3 Boost so we can add 3 MAG. 1 Boost is "left over" but it is not usable in this case.
This gives the Power a total MAG of 11 (i.e. 8+3 = 11)!
NOTE: Many of the power cards can be enhanced by Boosting, that's why Aspect Cards cards are pretty important!
The Aspect requires varies from power to power - so it is up to you how focused or broad you want your character to specialize. And even if you don't have the right Aspect to boost a power - don't worry, you can still use the power as written.