Tread safe: it can be used simultaneously in two different threads (running concurrently) without problem.
Usually, a class isn't thread-safe when it is using some static data, shared among all instances. So when thread changes this static data, it also acts on other instances, thus on other threads.

Re-usable: a quite vague concept... Since it is about the instance of a factory, I suppose it means one can use it several times. (?) Not knowing the code, it is hard to tell.
In other contexts, a re-usable code means it is flexible enough to be used in other contexts.

Thanks PhiLho. Here is the code for where i got this comment line. Any way code is working, i'm just curious.