The Boost feature allows you to prioritize goals so that more funds are added to them when Digit saves.
For example, if you had 3 active goals and Digit thought it could safely save $6 that day, each goal would get $2 allocated towards it. However, if you boosted one of those goals, you may see that the two non-boosted goals might get $1.50 each, and the boosted goal would get $3 added to it. It's still the same amount saved on a given day, with your selected goals getting priority.
If you have an end date set on your goal, you won't be able to Boost it, since the rate it's saving is already being controlled by that date.
To enable Boost:
1) Open the app and tap into the goal you'd like to boost
2) In your goal, tap Settings to bring up a menu. Then, choose Boost.
You'll see a lightening bold next to all of the goals that you have Boosted.
To disable Boost:
Repeat steps 1 & 2 above, then tap "Turn Boost Off"