In a chair cyclohexane, an axial substituent on C1 experiences steric (1,3-diaxial) repulsion with the axial groups on

The axial bonds on C1, C3 and C5 point to the same face of the ring and are parallel, so an axial group on C1 lies close to the axial groups on C3 and C5. These are the 1,3-diaxial interactions that make axial substituents less stable.