What does "the subgroups of $G$ form a chain" mean? I am being asked to show that: $G$ is a cyclic $p$-group $\iff$ its subgroups form a chain.
What does "its subgroups form a chain" mean? 
Please keep in mind that I am just asking for the meaning of that phrase.

The subsets of $G$ form a partially ordered set with respect to the inclusion $\subseteq$; the subgroups are a subset of this partially ordered set.
For any partiall ordered set $(S,\leq)$ a subset $C \subseteq S$ is called a chain if $C$ is totally ordered with respect to $\leq$.
So the subgroups of $G$ forming a chain means that the subgroups of $G$ are totally ordered with respect to the subgroup relation $\subseteq$.
