The structure of cyclohexane has important consequences for its reactivity, as we shall see.

The low energy form (strain free form) of cyclohexane is the chair:

The reason why this form is of lower energy can be seen if we look down any of the C-C bonds.  The following diagram is a Newman Projection of the areas indicated by the arrows above:

Clearly, both of these projections show that there is a staggered arrangement around each C-C bond, and therefore, repulsive interactions are minimized.

A consequence of this low energy form is that there are two distinct positions for the hydrogen atoms to occupy; axial, and equatorial.  The diagram below has had the hydrogen atom labels replaced with "ax" ("axial"), or "eq" ("equatorial") to indicate their position:

The higher energy conformation of cyclohexane is the boat form:

The positions of the hydrogen atoms have been omitted for clarity.  In this form, there are many eclipsing interactions, which contribute to its higher energy. 

However, in cyclohexane, the differences in energy are not sufficient to prevent ring-flipping.  This process (if it occurs rapidly enough, and it does in cyclohexane) will average out all of the axial and equatorial positions, so that they are effectively equivalent.

Ring-flipping occurs via a twist-boat conformation:

and converts all the axial positions to equatorial, and vice versa.

However, if the energy gap is sufficient, ring-flipping can be made very unlikely, and negligible. 

This can be achieved by the addition of a locking-group such as tertiary-butyl [C(CH₃)₃].

This makes the interconversion of the two conformational isomers (one with the tertiary-butyl group axial, and one with it equatorial) very slow.

As can be seen clearly from the diagram above, the equatorial isomer will have lower energy, as the axial isomer has unfavourable interactions between the tertiary-butyl group and the two axial hydrogen atoms indicated. 

These interactions are known as 1,3 diaxial interactions.  In the case of a very large group like tertiary-butyl, the interactions are so large, and so strongly disfavour the axial conformer, that the equatorial isomer almost completely dominates.