When you share a Post to another Team (or Teams), you are allowing multiple Teams to view and take action on that Post (comment, close, etc.) at the same time. It is truly a "shared" Post with all activity on that Post updating in real-time across all Teams with access to that Post.
Here's how to share a Post to another Team:
Click on the the Post you want to share, click on the "•••" icon, and select "Share".
In the window that pops up, select the teams you want to share the Post with (note: you can also remove Teams from this same window if you'd like).
Select who should be notified about this action on the next screen (if anyone). When complete, click "Share post".
And you're all set. That post will be shared with the Teams you selected!