Like sp3, sp2 hybrids are a combination of s and p atomic orbitals. In this case, you take one s and two p orbitals to make three sp2 orbitals. These three orbitals are arranged pointing out from the atomic center in a trigonal planar arrangement.
Thus, any atom with an atomic center that has a trigonal planar electronic geometry is sp2 hybridized.
Because only three of the four atomic orbitals are used to make the sp2 hybrid orbitals, there is one "extra" p orbital remaining on the atomic center. This p orbital can overlap with neighboring p orbitals to form a π bond. Thus, we have direct overlap of orbitals that leads to sigma bonding and "side-on" overlap that leads to pi bonding. Any double bond is described as one sigma bond plus one pi bond. This pi bond can only form if the p orbitals "line up" with each other. This means that double bonds cannot rotate, as rotation of the molecule around this axis effectively breaks this overlap and destroys the pi bond. It is not that it can't ever happen. It just requires a lot of energy (it is like breaking a bond).