A methyl group may rotate around the  axis. This is a free rotation only in the simplest cases like gaseous methyl chloride . In most molecules, the remainder R breaks the C symmetry of the  axis and creates a potential V(φ) that restricts the free motion of the three protons. For the model case of ethane , this is discussed under the name ethane barrier.
In condensed phases, neighbour molecules also contribute to the potential. Methyl group rotation can be experimentally studied using quasielastic neutron scattering.