Pinning an opportunity puts it on your own dashboard, so whatever is waiting on you is visible the moment you log in.
It applies to your admin profile only. Pinning is how you personalise your dashboard, not a setting that changes the opportunity itself.

The opportunity is then marked Pinned to dashboard, and it appears on your dashboard straight away.
The same menu. Click the three dots on the opportunity card and choose Unpin from dashboard.
Nothing is lost when you unpin. It only stops appearing on your dashboard, and the opportunity, its people and its shifts are untouched.
No. It applies to your admin profile and nothing else.
Your colleagues' dashboards do not change, and no user sees any difference. Two administrators in the same account can pin completely different opportunities.
Yes. Forms and sequences use the same Pin to dashboard action from their own pages, which is worth knowing if the thing you check every morning is an intake queue rather than an opportunity.