From VFXPedia

Bins Overview

Fusion's Bins provide an easy means of accessing commonly-used tools, settings, macros, compositions and footage. Use the bins to keep all media resources close at hand or to improve the collaborative workflow.

When adding an item to the bins, a link is created between the item and the bins. Fusion does not copy the file into its own cache or framestore so there is no need to dedicate large amounts of drive space to Fusion bins.