What UUID v4 means
A UUID is a 128-bit identifier. Version 4 reserves a few identifying bits and fills the remainder with random data, producing values such as 550e8400-e29b-41d4-a716-446655440000.
Collision probability
The random space is large enough that accidental collisions are extremely unlikely when UUIDs are generated correctly. A database should still enforce uniqueness when duplicates would be harmful.
Local generation
The identifiers are created with the browser cryptography API. Nothing is requested from a server.