A safe space is nothing more than a place (could be real or on the internet etc.) where nobody challenges your views of the world.
Example: Anyone who uses "triggered" unironically is really just saying "I'm too weak/stupid to defend my own view and I want a safe space". So they spend most of their time on tumblr, a notorious safe space for crybabies and degenerates alike.
[–] musicalbaboon ago
The basic idea is that a space is autonomous - ran by people from marginalised group, exclusively for people of marginalised group.