I'm a beginner here so I'd really appreciate an answer in simple English. Wikipedia defines Dihedral groups as a collection of rotational isometries $r, r^2, \ldots, r^n=e$ and reflection isometries $s, sr, sr^2, \ldots, sr^{n-1}$ satisfying $(sr)^2 =e$ of $n-$gon.
How are the smaller Dihedral groups, $D_1$ and $D_2$ defined and "what" are they rotational and reflection isometries of? Wikipedia directly starts mentioning relations like $D_1$ is $\mathbb{Z}_2= \mathbb{Z} /2\mathbb{Z}$ and $D_2$ is Klein group but what is $D_1$ and $D_2$, in the first place.
How do I view $D_1$ and $D_2$ as symmetries of $1-$gon and $2-$gon respectively? How does one picture a $1$ and $2$ sided polygon? Can anyone walk me through it?
MSE question 470570 is another related question but the OP and the person answering seem to already agree on what a $1-$gon is. I don't understand that $1−$gon or $2−$gon. A polygon is defined as closed plane figure with edges and vertices. What are edges and vertices in this case? What is a reflection here? What is a rotation? What do the elements in $D_1$ and $D_2$ look like?