SQlite Single-thread Mode and Transactional Locks

I am a bit confused between two concepts, namely, Threading modes of SQlite and Database locks. Is there a relation between the threading mode and the database locks? The documentation states "Single-thread. In this mode, all mutexes are disabled and SQLite is unsafe to use in more than a single thread at once.". Does it mean that no locks are applied in the case of Single-thread mode? What does it actually means to be "unsafe to use". Or are these entirely different concepts at different layers of abstractions and I am mixing them up!!