1,3-diaxial interaction
When a substituent on a cyclohexane chair sits in an axial position, pointing straight up over the ring, it does not point into empty space. Two carbons away around the ring — at positions 3 and 5 relative to it — sit two other axial bonds pointing up on the same face, and the three of them crowd into the same patch of space above the ring. The bumping between an axial group and those two same-side axial neighbors is the 1,3-diaxial interaction.
Geometrically these three axial positions form a little triangle hovering over one face of the chair. A hydrogen there is small and barely complains, but a larger group — a methyl, an isopropyl, a tert-butyl — runs into the two axial hydrogens (or groups) at the 3 and 5 carbons and is squeezed. This is a form of steric strain, and it is the real reason an axial substituent is higher in energy than the same substituent equatorial: an equatorial group escapes the crowd by pointing outward, where nothing is in its way.
Counting 1,3-diaxial interactions is the everyday tool for predicting ring conformations. Each axial methyl, for instance, suffers two 1,3-diaxial methyl-hydrogen contacts costing roughly 3.8 kJ/mol each (the same crowding as a gauche butane), which is why methylcyclohexane prefers its equatorial chair by about 7.5 kJ/mol. With a tert-butyl group the penalty is so large (over 20 kJ/mol) that the ring is essentially locked with that group equatorial — a trick chemists exploit to freeze a ring's shape.
An axial tert-butyl group suffers such severe 1,3-diaxial crowding (over 20 kJ/mol) that tert-butylcyclohexane is effectively locked in the chair with the tert-butyl equatorial.
Axial groups clash with the two axial bonds at carbons 3 and 5 on the same face — that crowding is what makes axial costly.
A 1,3-diaxial interaction is steric strain (groups colliding through space), the same kind as gauche butane — not torsional strain. It is the quantitative reason axial is disfavored.