Cyclohexane exists in two forms and these are chair and boat conformations. In the chair form, all the $C$ -H bonds on adjacent carbon atoms, are in the skew position. In the boat form, four of the $C$ -H bonds are skew $(1,2 ; 3,4 ; 4,5$ and $6,1)$ and two are eclipsed $(2,3$ and $5,6)$. At the same time, there will also be some bond opposition strain for these two pairs of eclipsed bonds and also steric repulsion between the hydrogen pointing towards each other at $1 $ and $ 4$ .
Hence, the total strain in the boat conformation is larger than that in the chair conformation and consequently the former is less stable than the latter.
