constructor() : variable() {} <- what does it do?

[Notsosuperhero]

I was reading up on the Half Life 2 SDK when I saw something I've never seen before.

class CMyLogicalEntity : public CLogicalEntity
{
public:
	DECLARE_CLASS(CMyLogicalEntity, CLogicalEntity);
	DECLARE_DATADESC();

	CMyLogicalEntity(void) : m_nCounter(0) {}

	void InputTick(inputdata_t &inputData);

private:

	int m_nThreshold;
	int m_nCounter;

	COutputEvent m_OnThreshold;
};

It has this class, which I know classes, but I don't know what this does:

CMyLogicalEntity(void) : m_nCounter(0) {}

what is that colon( there for, what does it do when you put that there?