The metals(Li,Na..) have one electron in thir outershell i.e. 2s1(Li) & 3S1(Na)..

So when atoms of (Li or Na) brought closer the bands form and they overlap..

Can any one describe why it happens only in metals

 

It doesn't just happen in metals. It happens in Si and Ge which are not metals, but semiconductors. In fact as quantum mechanical objects electrons have small finite probabilities of bing almost anywhere.