Do atoms in a molecule actually keep rotating freely in case they have sigma bond? While reading about cis-trans isomers I found the concept of free rotation of atoms.it says 2 atoms connected by sigma bond do not have trans sis isomer because of free rotation.while in case of pi bond free rotation is not possible. My question is : do we mean actual rotation of atoms in molecule when we say free rotation? Does the rotation happen spontaneously? What things are responsible for the rotation? Or is it just a concept to understand the concept of cis-trans isomers?
You really are referring to molecular conformations:


  Conformation, any one of the infinite number of possible spatial arrangements of atoms in a molecule that result from rotation of its
  constituent groups of atoms about single bonds.


Below are given two possible (out of many!) conformations of the ethane $\ce{C2H6}$ molecule:





The single $\sigma$ bond allows free rotation around the inter-nuclear axis of the two $\ce{C}$ atoms. One consequence is that around the single bond no cis-trans isomers can exist.

But if we add a $\pi$ bond, so we have a double bond and ethene $\ce{C2H4}$, then the methylene groups can no longer freely rotate about the bond.

That's because the electron probability density added by the $\pi$ bond doesn't lie on the inter-nuclear axis, but above and below it. Double bonds, and by extension also triple bonds, are rigid, with respect to rotation of the atoms they bond. This can give rise to cis-trans isomers.

Below: highly schematised rendition of the electron density of a $\pi$ bond: