A Decision is an action that can be taken by your team through review queues, or while exploring users, or automatically through a Workflows (see the Creating A Workflow Guide, for more information on this).
Decisions are also the mechanism in which you train Sift Science's Machine Learning Model to help your business fight fraud. You can create as many Decisions as possible to better align the Actions you take on your end with end results in Sift.
If you re looking to connect Decisions with business logic on your side, follow this tutorial: Decisions Webhooks