Radiator's reservoir issue

Hi Guys,

My radiator coolant is usually low well below the MAX line whenever i check it. But whenever i fill it up, and drive for a while, after parking and turning my car off, the coolant starts to leak from the reservoir. Today i noticed it comes out from the hole with a rubber on it just behind the reservoir's cap. it only happens on a hot day.

Can someone explain me what that hole is for? Is it for over flow? And would it be because i fill the reservoir to the max? Or something wrong with the reservoir.

Thanks

Radiator's reservoir issue