In practice, forks and threads are at odds with each other. If you try to fork while there are active threads, you will very likely run into bugs. Even if you didn't create the thread, some other library might have, so it can be very tricky in practice.

Multithreading is a widespread programming and execution model that allows multiple threads to exist within the context of one process. These threads share the process's resources, but are able to execute independently.