What does void Enter() = 0; mean in C#?

// IThread.h
class IThread
{
public:
    void BeginThread();
    virtual void ThreadMain() = 0;
};

// IThread.cpp
extern "C"
{
    static void __cdecl IThreadBeginThreadHelper(void* userdata)
    {
        IThread* ithread = reinterpret_cast< IThread* >(userdata);
        ithread->ThreadMain();
    }
}
void IThread::BeginThread()
{
    m_ThreadHandle = _beginthread(
                     &IThreadBeginThreadHelper,
                     m_StackSize, reinterpret_cast< void* >(this));
}

class Thread
{
    virtual void* run(void *params) = 0;

    void start(void * params)
    {
        this->params = params;
        pthread_create(&threadId, 0, &Thread::static_run, this);
    }

    static void * static_run(void * void_this)
    {
         Thread * thread_this = static_cast<Thread*>(void_this);
         return thread_this->run(thread_this->params);
    }

private:
    pthread_t threadId;
    void *params;
};

std::thread thread;

void start(void * params)
{
    thread = std::thread([this]{run(params);});
}