Equatorial Methyl
The methyl group (red) is equatorial in this conformation. This is the more stable of the two conformations that interconvert by the ring-flipping process.
Axial Methyl
The methyl group (red) is axial in this conformation. This is the less stable of the two conformations that interconvert by the ring-flipping process because of the steric interactions between the axial methyl group and the axial hydrogens (blue) on the same face of the ring (1,3-diaxial interactions). These interactions are similar to the steric interactions in gauche butane (see the structure below). This conformation is less stable than the conformation with the methyl group equatorial by 1.7 kcal/mol (7.1 kJ/mol), the "axial strain energy" for a methyl group. As a result, a sample of methylcyclohexane is composed 95% of molecules with the methyl group equatorial and 5% of molecules with the methyl group axial.
The axial strain energy for an isopropyl group (2.2 kcal/mol [9.2 kJ/mol]) is only slightly larger than that for a methyl group because the isopropyl group has a hydrogen pointed back over the ring to interact with the axial hydrogens. The axial strain energy for an tert-butyl group (4.9 kcal/mol [20.5 kJ/mol]) is much larger because the tert-butyl group must have a methyl group pointed back over the ring to interact with the axial hydrogens.
Gauche Interactions
The red methyl group and the blue ring carbon are gauche about the bond between the green carbons. There is a similar gauche interaction between the red methyl group and the gold ring carbon.