axial and equatorial positions
On a chair-shaped cyclohexane, each ring carbon holds two outward bonds, and they are not equivalent. One points straight up or down, parallel to an imaginary vertical axis through the ring — that is the axial position. The other points outward and slightly tilted, fanning out around the ring's equator like the rim of a hat — that is the equatorial position. Picture a globe: axial bonds aim at the poles, equatorial bonds sit near the equator.
The two positions alternate as you walk around the ring: up-axial, down-axial, up-axial, and so on, so axial bonds point alternately up and down on neighboring carbons. Each carbon's two bonds are one axial and one equatorial, and on any one carbon the axial and equatorial bonds point to opposite faces (if axial is up, equatorial tilts slightly down, and vice versa). This geometry is fixed by the chair's shape and is the same on every chair you will ever draw.
The distinction matters enormously for substituents. An equatorial group points out into open space, away from the rest of the ring, so it is roomy and comfortable. An axial group points up over (or down under) the ring, where it crowds the other axial groups on the same face. A bulky substituent therefore strongly prefers the equatorial spot, and this single preference — equatorial is roomy, axial is cramped — drives the conformation, and often the reactivity, of every substituted six-membered ring.
In methylcyclohexane the methyl strongly prefers the equatorial position; the equatorial chair is favored over the axial chair by about 7.5 kJ/mol, so roughly 95 percent of molecules sit equatorial.
Equatorial = roomy and favored; axial = crowded by the other axial groups on the same face.
'Equatorial' is not the same as 'in the plane of the ring' — it tilts up or down too. And axial-versus-equatorial is a conformational choice (set by the chair), not a fixed cis-trans relationship.