Explanation of $\ker (\bar{m}-\bar{id})^2 \cap \{x_0=1\}$

Question

Let $m$ be an isometry on $\mathbb{R}^2$ which is a composition of a reflection and a translation.

The way to find the axis of the isomtry is by solving:

$$\ker (\bar{m}-\bar{id})^2 \cap \{x_0=1\}$$

Could someone please briefly explain the reason for that equation?

Thanks in advance!