Cross-Progression allows you to synchronize your game progression and your inventory (DLCs, purchases, skins) on several platforms.
It's a combination of summed values and highest values. Items with a level are merged by taking the highest level from all linked accounts; items which can be counted are merged by taking the total number of items across all linked accounts.
At the moment, this is only available over Steam and Stadia.
You'll need to link your different platform accounts to your BHVR account to enable Cross-Progression.