What the purpose of imbue in C++?

I'm working with some code today, and I saw:

extern std::locale g_classicLocale;
class StringStream : public virtual std::ostringstream
{
 public:
        StringStream() { imbue(g_classicLocale); }
        virtual ~StringStream() {};
};

Then I came in face of imbue. What is the purpose of the imbue function in C++? What does it do? Are there any potential problems in using imbue (non-thread safe, memory allocation)?