ELI5: what is a safe space in practical terms?

I dont mean an abstract definition. But practically, what is a safe space? How does an organization create one and how does it enforce the safe-ness? Does an organization just verbally declare a room as a safe space? And what happens if two people make each other feel unsafe (eg a BLM activist and a KKK member both want to relax in a safe place)? Who leaves?

I come in good faith to learn