Notice that in both conformations all the bonds to the substituents on one carbon are staggered with respect to those on the adjacent carbons. Despite this, the structure on the right has higher potential energy than the one on the left. Before we look at why this is so, we need to expand our vocabulary: There are two types of hydrogens in cyclohexane, axial and equatorial. If you imagine the six carbon atoms of the ring to define the "equator" of the ring, those hydrogens that lie more or less along the "equator" are called equatorial, while those which are oriented vertically are called axial . Notice in Figure 4 that there are 6 axial hydrogens; 3 point up while the other 3 point down. Similarly, there are 6 equatorial hydrogens; 3 angle upwards, while the other 3 angle downwards. Click here for a demonstration on how to draw cyclohexane rings. Now back to methylcyclohexane.
The conformation of methylcyclohexane in which the methyl group is axial is less stable than its equatorial partner because of a phenomenon called 1,3-diaxial interactions. Basically, the axial methyl group is too close to the other two axial hydrogen atoms that are on the same side of the "equator". This causes increased electron-electron repulsions. To demonstrate this, diplay both models in the space filling format: select the Space Filling sub-menu from the Display Models menu. Notice the distance between the axial hydrogen atoms in the left-hand conformation. Compare that to the distance between one of the hydrogen atoms on the axial methyl group and one of the axial hydrogens on the ring of the right-hand model. The implication of "1,3" in the term 1,3-diaxial interactions is that the interactions occur between axial groups attached to every other atom, not between groups on adjacent atoms. In the right-hand model of methylcyclohexane there is a 1,3-diaxial interaction between the axial methyl group and both of the axial hydrogens.